harden(oidc): relax alg-downgrade IdP-bind check to intersection-empty (Keycloak compat)

Phase-10 live-IdP smoke (Keycloak 26.x via testcontainers-go) revealed
the IdP-bind alg-downgrade check was too strict for real-world IdPs.
6 of the integration tests in internal/auth/oidc/integration_keycloak*_test.go
were failing with:

  oidc: IdP advertises weak signing algorithms (HS*/none);
  refusing to use as defense against downgrade attacks: HS256

Keycloak 26.x (and several other real-world IdPs — Auth0 when HS-mode is
enabled, some Authentik configs) advertise EVERY alg they're capable of
in the discovery doc's id_token_signing_alg_values_supported field, even
when the realm only signs with RS256 in practice. Pre-fix the IdP-bind
check refused on ANY HS* or 'none' advertisement → no real Keycloak deploy
could ever bind a provider row, hence the integration-test failures.

The strict-deny check was defense-in-depth on top of the load-bearing
per-token alg-pin at sig-verify time (isDisallowedAlg, service.go L1177):
that check rejects every ID token whose JWS header carries an alg outside
DefaultAllowedAlgs, regardless of what the discovery doc advertises.
A forged HS256 token signed with the IdP's RS256 pubkey as HMAC secret
is rejected at sig-verify time → the actual algorithm-confusion attack
is closed by the per-token pin, NOT by the discovery-doc check.

Fix: relax the IdP-bind check to refuse only when the intersection of
advertised vs DefaultAllowedAlgs is EMPTY (the pathological all-weak-alg
IdP case). Keycloak (RS256 + HS256 advertised) now binds successfully;
an HS-only IdP still fails closed.

Changes:
- internal/auth/oidc/service.go: rewrite the alg-check loop at L1067 in
  getOrLoad / RefreshKeys to compute the intersection set; refuse only
  when no acceptable alg is advertised. ErrIdPDowngradeAdvertised
  docstring updated to reflect new contract. DefaultAllowedAlgs
  docstring + the package-level design-comment block at L40-72 updated
  with v2.1.0-relaxed semantics callouts.
- internal/auth/oidc/test_discovery.go: TestDiscovery dry-run validator
  rewritten to surface HS*/none alongside RS* as an informational note
  ('note: IdP advertises weak algorithms %v alongside acceptable ones')
  rather than a hard-fail error. HS-only / none-only still hard-fails.
- internal/auth/oidc/service_test.go: TestService_IdPDowngradeDefense_*
  tests updated. Renamed:
  - RejectsHSAdvertised → RS256PlusHS256_BindsSuccessfully (positive)
  - RejectsNoneAdvertised → RejectsHSOnlyAdvertised (intersection-empty)
  - RefreshKeys_CatchesPostLoadDowngrade rotated to HS-only post-load
- internal/auth/oidc/coverage_fill_test.go: TestTestDiscovery_AlgDowngradeDetected
  split into _HS256AlongsideRS256_BindsWithNote (positive, asserts note
  but no hard-fail) + _HSOnly_StillTrips_HardFail (intersection-empty).
- docs/operator/auth-threat-model.md: OIDC token-validation alg-allow-list
  section rewritten to call out the load-bearing-defense hierarchy
  (per-token pin first, IdP-bind check defense-in-depth) and document
  the v2.1.0 relaxation rationale.
- CHANGELOG.md: ### Security entry under Unreleased.

Verify: go test ./internal/auth/oidc/ -short PASS; gofmt clean; go vet
clean. The Keycloak integration tests should now pass when the operator
re-runs 'make keycloak-integration-test'.

CHANGELOG.md

@@ -2,6 +2,40 @@
 
 ## Unreleased
 
+### Security
+
+- **Alg-downgrade defense relaxed for Keycloak-shape IdPs (v2.1.0 pre-tag fix).**
+  Pre-fix, the IdP-bind alg-downgrade check at `internal/auth/oidc/service.go`
+  refused to load any OIDC provider whose discovery doc advertised HS256 /
+  HS384 / HS512 / `none` in `id_token_signing_alg_values_supported` —
+  even if RS256 was ALSO advertised. This broke binding against
+  Keycloak 26.x (and a handful of other real IdPs) which list every alg
+  the codebase is capable of in their discovery doc, regardless of which
+  one the realm actually signs with. The v2.1.0 Phase-10 live-IdP smoke
+  surfaced the regression: 6 testcontainers-Keycloak integration tests
+  failed with `oidc: IdP advertises weak signing algorithms (HS*/none); refusing to use as defense against downgrade attacks: HS256`.
+  **Fix:** the check now refuses only when the intersection of advertised
+  vs `DefaultAllowedAlgs` is EMPTY — an IdP advertising HS256 alongside
+  RS256 binds successfully, but an IdP advertising HS-only / none-only
+  still fails closed. The per-token alg pin at sig-verify time
+  (`isDisallowedAlg`, service.go ~L1177) remains the load-bearing defense
+  against the actual algorithm-confusion attack (forged HS256 token
+  signed with the IdP's RS256 pubkey as HMAC secret) — go-oidc/v3's
+  verifier rejects any token whose `alg` header isn't in the configured
+  allow-list, regardless of what the discovery doc claims. Updates:
+  `Service.getOrLoad` alg-check loop rewritten to compute intersection;
+  `ErrIdPDowngradeAdvertised` docstring reflects new semantics;
+  `TestDiscovery` dry-run validator surfaces HS*/none alongside RS* as
+  an informational note (not a hard fail); `docs/operator/auth-threat-model.md`
+  alg-allow-list section updated to call out the load-bearing-defense
+  hierarchy. Tests: `TestService_IdPDowngradeDefense_RS256PlusHS256_BindsSuccessfully`
+  (positive — Keycloak-shape) + `TestService_IdPDowngradeDefense_RejectsHSOnlyAdvertised`
+  (negative — pathological intersection-empty case) +
+  `TestService_RefreshKeys_CatchesPostLoadDowngrade` updated to assert
+  intersection-empty post-rotation; `TestTestDiscovery_AlgDowngrade_HS256AlongsideRS256_BindsWithNote`
+  + `TestTestDiscovery_AlgDowngrade_HSOnly_StillTrips_HardFail` pin the
+  dry-run validator's new behavior.
+
 ### Tests
 
 - **Vitest coverage for the 2026-05-10/11 GUI batch (Audit 2026-05-11 Fix 12).**