docs(decisions): add ADR-0018 — environment configuration strategy

Records the per-environment config strategy for both apps:

SPA — Angular `environment.ts` + project.json `fileReplacements`.
Build-time substitution; no runtime config fetch (rejected for
the LCP/TTFB cost it would add and the deploy-time HTML rewrite
the alternatives need). Concretely cleans up the hard-coded
URLs we left in observability/tracing.ts and home-status.service.
ts ("hard-coded for v1 — env-config when it lands" comments).

BFF — `process.env` + small per-key boot-time validators (the
shape `apps/portal-bff/src/config/check-database-url.ts`
already follows). `@nestjs/config` rejected as too heavy for the
current key count.

Audit log connection — formalises the `AUDIT_DATABASE_URL` split
that ADR-0013 §"Wired as features land" already pointed at: when
the env var is set, the AuditModule instantiates a second Prisma
client with audit_writer-only credentials (defense in depth);
when unset, the dev fallback (shared pool + `SET LOCAL ROLE
audit_writer` per transaction) keeps working. A boot-time UPDATE-
rejection self-test runs against the dedicated pool when
configured — refuses to start if the pool can mutate the audit
table.

Cross-references in the §Confirmation list pull together the
env-var-as-boot-gate items already pre-figured in ADR-0009 /
ADR-0010 / ADR-0012 / ADR-0013 / ADR-0014, so the validator
pattern is the single landing place when those features ship.

Doc index and CLAUDE.md updated; no code changes in this PR.

CLAUDE.md

@@ -46,6 +46,7 @@ The structural, security, observability, and quality choices are recorded as ADR
 - **CI/CD:** **Gitea Actions** (level-2 implementation; will be superseded by a GitLab migration ADR within 6-18 months). Trunk-based with squash-merge, branch protection on `main`, all CI gates blocking. Thin YAML — orchestration logic lives in `package.json` scripts (`ci:check`, `ci:scan`, `ci:commits`) and Nx targets, runnable locally. Gates: format / lint / type-check / test / build / audit / secret-scan / commit-lint, plus `a11y` (per ADR-0016) and future `perf`. Self-hosted `act_runner` on-prem. Conventional Commits validated locally (hook) and in CI (defense in depth). Required reviewer count = 0 in v1, raised to ≥1 once a second contributor joins. Signed commits recommended, revisited at GitLab migration — see [ADR-0015](docs/decisions/0015-cicd-gitea-actions.md).
 - **Accessibility:** **WCAG 2.2 AA baseline + targeted AAA** on criteria with high impact for APF's user base (1.4.6 Contrast Enhanced, 2.2.3 No Timing, 2.3.3 Animation, 3.1.5 Reading Level, 1.4.8 Visual Presentation, 2.4.9 Link Purpose, 3.3.5 Help). RGAA 4.1 alignment for French audit. UI stack: **Angular CDK + TailwindCSS** (spartan-ng _library_ deferred until it reaches 1.0.0; v1 components are written in-house in `libs/shared/ui/` on Angular CDK, applying the spartan-ng _philosophy_ of headless primitives + utility CSS + copy-paste). User-preferences panel (contrast / text size / motion / spacing / cognitive simplification / reading focus) persisted in session. Tooling: `@angular-eslint/template/*` lint, `@axe-core/playwright` e2e (blocking on critical/serious), token-contrast CI check, touch-target check (44×44 min). Manual testing cadence with APF's internal user panel before each major release. Public accessibility statement page at `/accessibility` and `/accessibilite` — see [ADR-0016](docs/decisions/0016-accessibility-baseline-wcag-aa-targeted-aaa.md).
 - **Performance budgets:** Core Web Vitals at Google "Good" thresholds (LCP ≤ 2.5 s, INP ≤ 200 ms, CLS ≤ 0.1, TBT ≤ 200 ms, TTFB ≤ 800 ms), Lighthouse Performance ≥ 90 on critical routes. **Lighthouse CI** (`@lhci/cli`) runs in CI with median-of-3 mitigation, blocking on threshold breach. Angular bundle `budgets` (`type: "error"`): initial ≤ 300 KB gzip, lazy chunks ≤ 100 KB gzip. BFF p95/p99 SLOs per endpoint family observed via OTel (advisory in CI, alerting in prod). Weekly scheduled Lighthouse run on prod env. **a11y wins over perf** when they conflict — see [ADR-0017](docs/decisions/0017-performance-budgets-lighthouse-ci.md).
+- **Environment configuration:** SPA per-environment values via Angular `environment.ts` + `fileReplacements` at build time (no runtime config-fetch). BFF reads `process.env` directly with small per-key boot-time validators (no `@nestjs/config` overhead at this scale). The audit log uses a separate `AUDIT_DATABASE_URL` connection pool in production (`audit_writer`-only login, defense in depth) and falls back to the shared pool + `SET LOCAL ROLE` in dev — see [ADR-0018](docs/decisions/0018-environment-configuration-strategy.md).
 - **Local quality gates:** Husky + lint-staged + commitlint with Conventional Commits — see [ADR-0007](docs/decisions/0007-pre-commit-hooks-and-conventional-commits.md).
 - **Runtime:** Node.js latest LTS major.