feat(portal-bff): signed-assertion strategy + /.well-known/jwks.json

Second half of the DownstreamApiClient + OBO chantier per ADR-0014.
Ships the signed-assertion strategy (non-Entra downstreams) and the
JWKS publishing endpoint as testable primitives. The framework
around them (DownstreamApiClientFactory, cockatiel, audience
pre-check, error translation) still waits for the first concrete
integration per the ADR's "until then" clause.

What lands

- assertJwksConfig (config/check-jwks-config.ts):
  - Reads the PEM private key once at boot, refuses missing /
    unreadable / weak material (RSA < 2048, Ed25519, unknown key
    type). Derives the JOSE algorithm (RS256 / ES256 / ES384) from
    the key shape so neither the strategy nor the JWKS controller
    has to re-decide on the hot path.
  - Validates BFF_JWKS_KID against [A-Za-z0-9_-]{4,128} so the
    value lives unescaped in JWT headers + JWKS payloads.
  - Wired in main.ts alongside the other assertX() validators.

- BffSigningKey (downstream/bff-signing-key.ts):
  - Singleton holding { config: JwksConfig, publicJwk: JWK }.
    publicJwk is derived from the private key via `jose.exportJWK`
    on a public KeyObject — no private material leaks through.
  - DI token BFF_SIGNING_KEY wires both consumers (strategy +
    controller) to the same source of truth.

- SignedAssertionStrategy (downstream/strategies/signed-assertion.strategy.ts):
  - Wraps `jose.SignJWT` with the ADR-0014 claim shape: iss,
    sub, aud, audience (workforce|customer), claims (curated
    subset), trace_id, iat, exp.
  - 60 s TTL hard-coded — the ADR mandates it; cache disabled
    because the savings on a 60 s JWT would be marginal and a
    cache would let replayed assertions linger past their TTL.
  - kid header matches the JWKS so a downstream picks the right
    key during rotation.
  - Supports RS256 / ES256 / ES384 transparently — picks the alg
    the validator derived at boot.

- JwksController (downstream/jwks.controller.ts):
  - GET /.well-known/jwks.json returns { keys: [<single jwk>] }.
  - main.ts excludes /.well-known/* from the global /api prefix so
    the route lands at the bare root per RFC 8615.
  - No auth gate (the JWKS is the verification anchor — gating it
    would defeat the purpose). Read-only, so the CSRF middleware's
    GET-exempt path already handles it.

Configuration

- Generate a key:
    mkdir -p apps/portal-bff/.secrets && \
    openssl genpkey -algorithm RSA -pkeyopt rsa_keygen_bits:3072 \
      -out apps/portal-bff/.secrets/jwks.pem
- BFF_JWKS_PRIVATE_KEY_PATH (path to the PEM)
- BFF_JWKS_KID (URL-safe id, 4..128 chars)
- Both mandatory at boot.
- `apps/portal-bff/.secrets/` is matched by the repo's existing
  *.pem / *.key gitignore patterns.

Deps

- jose@^6 added as a direct dep (was transitive). Pinned at the
  workspace root since the BFF is the only consumer today and the
  package isn't part of the Angular bundle graph.
- jest.config.cts: jose ships ESM-only, so its node_modules path
  is removed from transformIgnorePatterns. The pattern walks
  pnpm's deep `.pnpm/` layout — anything under /node_modules/ that
  also contains `jose` somewhere in the path gets transformed.

Tests: +24 specs (env validators 11, signing key 4, strategy 6,
controller 3).

Out of scope (deferred per ADR-0014 "until then"):
- DownstreamApiClientFactory + per-service typed config.
- cockatiel resilience composition.
- Audience pre-check at the call site.
- Error translation tables.
- OTel custom spans `downstream.<service>.<verb>.<path>`.
- The framework wiring that calls SignedAssertionStrategy.sign()
  + attaches the `X-User-Assertion` + ServiceCredential auth
  header to outbound HTTP requests.
- Key rotation (the JWKS lists one key for now; rotation chantier
  adds a second entry + a window-based eviction policy).

These land alongside the first concrete integration so the
framework shape is validated against a real consumer.

apps/portal-bff/src/downstream/jwks.controller.ts

@@ -0,0 +1,38 @@
+import { Controller, Get, Inject } from '@nestjs/common';
+import type { JWK } from 'jose';
+import { BFF_SIGNING_KEY, type BffSigningKey } from './bff-signing-key';
+
+/**
+ * `GET /.well-known/jwks.json` — publishes the BFF's public key
+ * material so downstream services can verify `X-User-Assertion`
+ * JWTs minted by `SignedAssertionStrategy` per
+ * [ADR-0014](../../../../docs/decisions/0014-downstream-api-access-obo-pattern.md)
+ * §"Service strategy".
+ *
+ * v1 publishes a single key. When the rotation chantier ships,
+ * `keys` will hold both the current and the previous public JWKs
+ * so a downstream that cached the previous one keeps verifying
+ * during the cut-over window. The shape is JWKS-canonical so
+ * existing JOSE clients on the downstream side just point at the
+ * URL and work.
+ *
+ * **Routing** — the controller's `@Controller('.well-known/jwks.json')`
+ * combined with `main.ts`'s `setGlobalPrefix('api', { exclude:
+ * [/^\.well-known/] })` lands the route at the bare-root path
+ * (`/.well-known/jwks.json`), which is where the well-known URI
+ * convention places it (RFC 8615).
+ *
+ * **No auth / no CSRF.** Public by design — the JWKS is the
+ * downstream's verification anchor; gating it would defeat the
+ * purpose. The double-submit CSRF middleware already exempts GET
+ * methods so the route comes out clean.
+ */
+@Controller('.well-known/jwks.json')
+export class JwksController {
+  constructor(@Inject(BFF_SIGNING_KEY) private readonly key: BffSigningKey) {}
+
+  @Get()
+  jwks(): { keys: readonly JWK[] } {
+    return { keys: [this.key.publicJwk] };
+  }
+}