diff --git a/docs/README.md b/docs/README.md
index ac96a0a..5d70a6d 100644
--- a/docs/README.md
+++ b/docs/README.md
@@ -15,6 +15,10 @@ This is the entry point to all project documentation. It is maintained automatic
- [development.md](development.md) — repo layout, prerequisites, initial setup, daily commands, conventional commit cycle. Day-to-day reference for working on the project.
+### Architecture
+
+- [architecture.md](architecture.md) — cross-cutting Mermaid diagrams: C4 system context, C4 containers, Nx module boundaries, CI/CD pipeline. Single-decision diagrams (auth sequence, ERD, etc.) live inline in their ADR.
+
### Onboarding & environment
Setup guides for new contributors:
@@ -23,10 +27,6 @@ Setup guides for new contributors:
- [setup/02-dev-web-stack.md](setup/02-dev-web-stack.md) — Node via nvm, pnpm via corepack, Docker
- [setup/03-angular-nx-monorepo.md](setup/03-angular-nx-monorepo.md) — Angular + Nx monorepo bootstrap
-### Architecture
-
-_Empty — to be populated as the project grows._
-
### Operations & runbooks
_Empty — to be populated when we deploy._
diff --git a/docs/architecture.md b/docs/architecture.md
new file mode 100644
index 0000000..5cf3ecd
--- /dev/null
+++ b/docs/architecture.md
@@ -0,0 +1,196 @@
+# Architecture diagrams
+
+Visual cross-cuts of `apf_portal`'s architecture. Each diagram summarises decisions that are spread across several ADRs and exists to help a contributor (or auditor, RSSI, IT contact) build a mental model fast. Decisions themselves live in [decisions/](decisions/); diagrams reflect the current accepted state.
+
+Diagrams are written in [Mermaid](https://mermaid.js.org/) — text in markdown, rendered natively by Gitea, GitHub, and most IDE markdown viewers. Updating a diagram is an ordinary text edit reviewable in PR.
+
+When a diagram is the _content_ of a single decision (e.g. a sequence diagram that captures a flow described by one ADR), it lives **inside that ADR**, not here. This file holds the _cross-cutting_ views.
+
+---
+
+## 1. System context (C4 level 1)
+
+The portal as a black box, with the actors that interact with it and the external systems it depends on.
+
+Sources: [ADR-0008](decisions/0008-identity-model-entra-workforce-dual-audience.md) (identity model), [ADR-0014](decisions/0014-downstream-api-access-obo-pattern.md) (downstream APIs), [ADR-0013](decisions/0013-audit-trail-separated-postgres-append-only.md) (audit access).
+
+```mermaid
+flowchart TB
+ workforce([Workforce user
employee])
+ customer([Customer user
external
future])
+ it([IT / Identity admin])
+ rssi([RSSI / SOC analyst])
+
+ portal[apf_portal
web portal centralising
access to existing apps]
+
+ entra[(Microsoft Entra ID
workforce tenant
+ M365 Developer dev tenant)]
+ extid[(Microsoft Entra External ID
customer tenant
future)]
+ downstream[(Downstream APIs
existing applications
integrated by the portal)]
+
+ workforce -->|browser HTTPS| portal
+ customer -.->|browser HTTPS
future| portal
+ portal -->|OIDC Auth Code + PKCE| entra
+ portal -.->|OIDC Auth Code + PKCE
future| extid
+ portal -->|OBO or signed assertion| downstream
+ it -->|configures
tenant + Conditional Access
+ B2B invitations| entra
+ rssi -->|reads audit events
via audit_reader role| portal
+
+ classDef future stroke-dasharray: 5 5,opacity:0.7
+ class customer,extid future
+```
+
+Dashed = future scope (not v1). The customer audience is _designed for_ but not implemented (per the dual-audience design in ADR-0008); it is shown to make the future expansion legible.
+
+---
+
+## 2. Containers (C4 level 2)
+
+The runtime artefacts and their conversations. One step deeper than system context: what is actually deployed.
+
+Sources: [ADR-0002](decisions/0002-adopt-nx-monorepo-apps-preset.md) (Nx layout), [ADR-0004](decisions/0004-frontend-stack-angular-csr-zoneless-signals.md) (Angular SPA), [ADR-0005](decisions/0005-backend-stack-nestjs.md) (NestJS BFF), [ADR-0006](decisions/0006-persistence-postgresql-prisma.md) (Postgres + Prisma), [ADR-0009](decisions/0009-auth-flow-oidc-pkce-msal-node.md) (auth flow), [ADR-0010](decisions/0010-session-management-redis.md) (sessions in Redis), [ADR-0012](decisions/0012-observability-pino-opentelemetry.md) (OTel collector), [ADR-0014](decisions/0014-downstream-api-access-obo-pattern.md) (downstream calls).
+
+```mermaid
+flowchart TB
+ user([Workforce user])
+
+ subgraph Browser["Browser"]
+ spa["portal-shell
Angular 21 SPA
zoneless / Signals / Tailwind 4"]
+ end
+
+ subgraph OnPrem["On-prem deployment"]
+ bff["portal-bff
NestJS 11 / Node 24
Express adapter"]
+ pg[("Postgres 17
schemas: public + audit
RLS for dual-audience")]
+ redis[("Redis
sessions + OBO token cache
AES-256-GCM at rest")]
+ otel["OTel Collector
local sidecar
(OTLP → backend, future)"]
+ end
+
+ entra[(Microsoft Entra ID)]
+ downstream[(Downstream APIs)]
+
+ user -->|HTTPS| spa
+ spa -->|"HTTPS
__Host-portal_session
X-CSRF-Token
traceparent"| bff
+ spa -. "OTLP / HTTP
(browser spans)" .-> otel
+ bff -->|"OIDC Auth Code + PKCE
via @azure/msal-node"| entra
+ bff -->|Prisma 7| pg
+ bff -->|"ioredis
(opaque session id
→ encrypted blob)"| redis
+ bff -->|"OBO token (Entra-protected)
or signed X-User-Assertion JWT
+ traceparent"| downstream
+ bff -. "OTLP / HTTP
(server spans + Pino logs)" .-> otel
+```
+
+Notes embedded in the diagram:
+
+- The SPA carries no token. Only the opaque session id cookie (`__Host-portal_session`) plus the CSRF cookie (`__Host-portal_csrf`, read by JS for double-submit).
+- `traceparent` (W3C) propagates from the browser through the BFF to the downstream APIs — same `trace_id` end-to-end.
+- The OTel Collector is the only piece coupled to the eventual on-prem observability backend. Choice of backend (Grafana Loki + Tempo, ELK, …) is deferred to the on-prem infrastructure ADR.
+
+---
+
+## 3. Nx module boundaries
+
+Which projects are allowed to depend on which. Encoded in `eslint.config.mjs` via `@nx/enforce-module-boundaries` with the `depConstraints` from [ADR-0003](decisions/0003-workspace-and-app-naming-convention.md).
+
+Each project carries two tags:
+
+- **`scope:`** — `portal-shell`, `portal-bff`, or `shared` (for libs consumable by both apps).
+- **`type:`** — `app`, `feature`, or `shared`.
+
+A dependency is allowed only if both axes permit it.
+
+```mermaid
+flowchart LR
+ subgraph Apps["apps · type:app"]
+ shell["portal-shell
scope:portal-shell"]
+ bff["portal-bff
scope:portal-bff"]
+ end
+
+ subgraph FeatureLibs["libs · type:feature"]
+ fauth["feature-auth
scope:portal-shell"]
+ end
+
+ subgraph SharedShellLibs["libs · type:shared · scope:portal-shell"]
+ sui["shared-ui
Angular components
(spartan-style on CDK)"]
+ end
+
+ subgraph SharedAnyLibs["libs · type:shared · scope:shared"]
+ stokens["shared-tokens
design tokens (a11y)"]
+ sutil["shared-util
cross-runtime TS helpers"]
+ end
+
+ shell --> fauth
+ shell --> sui
+ shell --> stokens
+ shell --> sutil
+
+ bff --> stokens
+ bff --> sutil
+
+ fauth --> sui
+ fauth --> stokens
+ fauth --> sutil
+
+ sui --> stokens
+
+ classDef forbidden stroke:#c00,stroke-dasharray: 4 4
+```
+
+Forbidden by the depConstraints (and lint-enforced) — examples:
+
+- `portal-bff` ⟶ `shared-ui` (back imports Angular code: scope clash).
+- `portal-bff` ⟶ `feature-auth` (back imports a frontend feature).
+- `shared-tokens` ⟶ `shared-ui` (a `type:shared` lib cannot reach a `type:shared` lib in a narrower scope, nor a feature lib).
+- Any lib ⟶ any app.
+
+---
+
+## 4. CI/CD pipeline
+
+How a change moves from a developer's keyboard to `main`. Reflects [ADR-0007](decisions/0007-pre-commit-hooks-and-conventional-commits.md) (local hooks), [ADR-0015](decisions/0015-cicd-gitea-actions.md) (Gitea Actions, trunk-based + squash-merge), [ADR-0016](decisions/0016-accessibility-baseline-wcag-aa-targeted-aaa.md) (a11y gate), [ADR-0017](decisions/0017-performance-budgets-lighthouse-ci.md) (perf gate).
+
+```mermaid
+flowchart TB
+ edit[Edit on feat/*, fix/*, chore/*, docs/* branch]
+ pre[".husky/pre-commit
lint-staged → prettier on staged"]
+ msg[".husky/commit-msg
commitlint → Conventional Commits"]
+ push[git push origin feat/*]
+ pr[Open PR → main]
+
+ subgraph PRJobs["CI on PR · .gitea/workflows/ci.yml · all blocking"]
+ direction LR
+ j1["check
nx affected -t
format / lint / test / build"]
+ j2["scan
pnpm audit + Trivy + gitleaks"]
+ j3["commits
commitlint on PR range"]
+ j4["perf
Lighthouse CI
CWV thresholds (ADR-0017)"]
+ j5["a11y
axe-core via Playwright
(placeholder until first screens)"]
+ end
+
+ protection{"Branch protection on main
· all 5 jobs green
· ≥0 reviewers (v1, →≥1 with 2nd contributor)
· linear history
· no direct push, no force push"}
+ squash[Squash-merge
subject = Conventional Commits
(becomes the commit on main)]
+ tag[Tag vX.Y.Z]
+ release[".gitea/workflows/release.yml
(stub today, populated with on-prem deploy ADR)"]
+
+ edit --> pre --> msg --> push --> pr
+ pr --> PRJobs --> protection -->|all green| squash --> tag --> release
+
+ cron["Weekly cron · Mon 04:00 UTC"]
+ sched[".gitea/workflows/security-scheduled.yml
full-tree Trivy + gitleaks
+ Lighthouse on prod (when LHCI_PROD_URL set)"]
+ cron --> sched
+```
+
+Note the parallelism: the five PR jobs run **in parallel**. The diagram shows the gate as one square because branch protection requires _all five_ before squash-merge is allowed.
+
+---
+
+## To be added
+
+As features land, the following diagrams will be added — either here, or inline in the ADR they belong to (per the convention stated at the top: _cross-cutting → here, single-decision → in the ADR_).
+
+| Diagram | Where it will land | Triggered by |
+| ------------------------------------------------------------------------------------------ | ---------------------------------------------------------------------------------------------- | ----------------------------------------------------- |
+| **OIDC Auth Code + PKCE sequence** | inline in [ADR-0009](decisions/0009-auth-flow-oidc-pkce-msal-node.md) | already useful — added at the same time as this file |
+| **Trace context propagation** (SPA → BFF → DB → downstream) | here | first observability instrumentation lands |
+| **Dual-audience flow** (token validation, claim → enum, RLS filtering) | here or split between ADRs 0008/0009/0013 | first authz code that touches the audience |
+| **Step-up MFA flow** (claims challenge round-trip) | inline in [ADR-0011](decisions/0011-mfa-enforcement-entra-conditional-access.md) | first `@RequireMfa()` route |
+| **Database ERD** | inline in [ADR-0006](decisions/0006-persistence-postgresql-prisma.md) or a dedicated `data.md` | first business model in `schema.prisma` |
+| **Audit event lifecycle** (writer → store → archiver → reader) | inline in [ADR-0013](decisions/0013-audit-trail-separated-postgres-append-only.md) | audit module shipped |
+| **Downstream call lifecycle** (audience pre-check → strategy → cache → resilience → trace) | inline in [ADR-0014](decisions/0014-downstream-api-access-obo-pattern.md) | first downstream integration |
+| **Trust boundaries** (security review view) | here | when the RSSI requests a security architecture review |