✨ feat(admin): GET /api/v1/admin/jwt/secrets — metadata-only introspection

Closes the missing piece of ADR-0021's admin surface. Was referenced by
the @todo BDD scenarios in features/jwt/jwt_secret_retention.feature
since PR #41 but never wired up.

Security-first design:
- Endpoint returns metadata ONLY: is_primary, created_at_unix,
  expires_at_unix?, age_seconds, is_expired, secret_sha256 (8-byte
  prefix as fingerprint). The secret VALUE is intentionally never
  returned — exposing it via API would defeat the retention/rotation
  infrastructure. The fingerprint is enough for ops correlation in
  logs without leak surface.
- Routed under /api/v1/admin/jwt/secrets. The existing admin auth
  middleware (POST endpoints below) gates GET in the same way — same
  router subtree.

Plumbing:
- New JWTSecretInfo struct in pkg/user/user.go (metadata-only).
- AuthService.ListJWTSecretsInfo() interface method.
- userServiceImpl.ListJWTSecretsInfo() implementation: calls
  GetAllValidSecrets, computes age + fingerprint, returns view.
- handleListJWTSecrets in pkg/user/api/admin_handler.go.
- Documentation/API.md updated with full schema + security note.

Tests:
- TestListJWTSecretsInfo_ReturnsMetadataOnlyNotSecretValues in
  pkg/user/jwt_manager_test.go covers GetAllValidSecrets exclusion of
  expired secrets (the underlying primitive). go test -race passes.
- Full BDD suite (auth/config/greet/health/info/jwt) green.

@todo BDD scenarios in features/jwt/jwt_secret_retention.feature can
now be activated in a follow-up PR — left as @todo for review.

pkg/user/user.go

@@ -52,6 +52,24 @@ type AuthService interface {
 	// the count of removed non-primary expired secrets. Useful for tests
 	// driving cleanup synchronously.
 	RemoveExpiredJWTSecrets() int
+	// ListJWTSecretsInfo returns metadata about every currently-tracked JWT
+	// secret WITHOUT exposing the secret values. Used by the admin
+	// introspection endpoint and BDD tests verifying cleanup behavior.
+	// Order is preserved from internal storage (insertion order).
+	ListJWTSecretsInfo() []JWTSecretInfo
+}
+
+// JWTSecretInfo is a non-sensitive metadata view of a JWT secret.
+// The secret VALUE is intentionally NOT included — exposing it via an
+// API endpoint, even an admin one, would defeat the point of the
+// retention/rotation infrastructure.
+type JWTSecretInfo struct {
+	IsPrimary     bool   `json:"is_primary"`
+	CreatedAtUnix int64  `json:"created_at_unix"`
+	ExpiresAtUnix *int64 `json:"expires_at_unix,omitempty"`
+	AgeSeconds    int64  `json:"age_seconds"`
+	IsExpired     bool   `json:"is_expired"`
+	SecretSHA256  string `json:"secret_sha256"` // first 16 hex chars of sha256 — fingerprint, not the secret
 }
 
 // UserManager defines interface for user management operations