✨ feat(auth): JWT secret retention policy + automatic cleanup loop (ADR-0021)

Implements the cleanup half of ADR-0021 (which had only config infrastructure
landed). Non-primary expired secrets are removed by a goroutine that runs at
auth.jwt.secret_retention.cleanup_interval (default 1h). Primary secret is
never removed regardless of expiration — invariant preserved.

Changes:
- pkg/user/jwt_manager.go : add sync.Mutex protection; add
  RemoveExpiredSecrets() int and StartCleanupLoop(ctx, interval) methods.
  Reset() now also cancels any running cleanup goroutine.
- pkg/user/auth_service.go : delegate to manager via new AuthService methods
  StartJWTSecretCleanupLoop and RemoveExpiredJWTSecrets.
- pkg/user/user.go : extend AuthService interface accordingly.
- pkg/server/server.go Run() : start cleanup loop tied to rootCtx so it
  stops on graceful shutdown.
- pkg/jwt/* : same treatment on the secondary (less-used) implementation
  for consistency.
- adr/0021-jwt-secret-retention-policy.md : Status → Implemented + fix
  numbering (was incorrectly "10.").

Tests:
- 4 new unit tests in pkg/user/jwt_manager_test.go covering
  RemoveExpiredSecrets (expired removed, primary preserved, future kept)
  and StartCleanupLoop (fires + stops on context cancel).
- go test -race ./pkg/user/... passes.
- Full BDD suite (auth/config/greet/health/info/jwt) still green.
- BDD scenarios at @todo / @skip remain so — they require an admin
  endpoint /api/v1/admin/jwt/secrets which is explicitly out of scope.

Verifier verdict: APPROVE_WITH_NITS — StartCleanupLoop is 34 lines (just
over the 30-line guideline); 2 time.Sleeps in TestStartCleanupLoop_FiresAndStops
are justified by the goroutine-timing nature of the test.

pkg/jwt/jwt.go

@@ -24,13 +24,25 @@ type JWTSecret struct {
 	ExpiresAt *time.Time // Optional expiration time
 }
 
-// JWTSecretManager manages multiple JWT secrets for rotation
+// JWTSecretManager manages multiple JWT secrets for rotation.
+// Secrets can carry an optional expiration; the cleanup loop removes them
+// after expiry while always preserving the primary secret (ADR-0021).
 type JWTSecretManager interface {
 	AddSecret(secret string, isPrimary bool, expiresIn time.Duration)
 	RotateToSecret(newSecret string)
 	GetPrimarySecret() string
 	GetAllValidSecrets() []JWTSecret
 	GetSecretByIndex(index int) (string, bool)
+
+	// RemoveExpiredSecrets drops every non-primary secret whose ExpiresAt is
+	// non-nil and in the past. Returns the count of secrets removed.
+	// The primary secret is never removed regardless of expiration.
+	RemoveExpiredSecrets() int
+
+	// StartCleanupLoop spawns a goroutine that calls RemoveExpiredSecrets at
+	// the given interval. Stops when the context is cancelled. Safe to call
+	// once at startup; calling again replaces the previous loop's context.
+	StartCleanupLoop(ctx context.Context, interval time.Duration)
 }
 
 // JWTService defines interface for JWT operations