RBAC Phase 1: membership authority endpoint + fail-closed API auth (#14)

Implements the model-B2 membership authority (ratified compliance auth design) and inbound token verification.

- GET /v1/users/{id}/memberships: Keycloak supplies user->tenant links + org_roles (attribute projection until the realm migrates to Organizations); registered tenants override status/plan/products from registry tables (source: registry|keycloak). Closes the M5.2-deferred org_roles stub.
- New internal/authn: OIDC discovery + JWKS verification (go-oidc/v3), audience tenant-registry. /healthz + /readyz stay PUBLIC_EXPLICIT; all other routes INTERNAL_SERVICE_ONLY. AUTH_ENABLED=true refuses to start on incomplete config (fail-closed); false (dev default) keeps current behavior.
- Full suite green incl. postgres testcontainers; openapi.yaml updated (contract test passes).

Activation is a separate step (Auth-5): requires the orca-infra env merge and a portal service token (portal currently calls with no auth).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Sharang Parnerkar <30073382+mighty840@users.noreply.github.com>
Reviewed-on: #14

internal/keycloak/mock.go

@@ -52,6 +52,24 @@ func (m *Mock) CreateOrgAndInvite(_ context.Context, in InviteInput) (*InviteRes
 	}, nil
 }
 
+// Memberships returns the last-synced Claims for userID as its single
+// membership, mirroring the attribute-projection behavior of the real
+// adapter. Unknown users yield ErrUserNotFound.
+func (m *Mock) Memberships(_ context.Context, userID string) ([]Claims, error) {
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	if m.FailNext != nil {
+		err := m.FailNext
+		m.FailNext = nil
+		return nil, err
+	}
+	c, ok := m.Claims[userID]
+	if !ok {
+		return nil, ErrUserNotFound
+	}
+	return []Claims{c}, nil
+}
+
 func (m *Mock) SyncClaims(_ context.Context, userID string, c Claims) error {
 	m.mu.Lock()
 	defer m.mu.Unlock()