§22 M3-backend Plan B (2/2, read path): per-project RFC serving (v0.37.0)

A second project's corpus now renders under /p/<id>/, isolated by its own
slug namespace. Completes step (1) "RFC app supports multiple projects" for
the read path.

- api.py: GET /api/projects/{pid}/rfcs (scoped catalog) + /{slug} (entry by
  (project_id,slug)), behind the §22.5 read gate. Unscoped /api/rfcs stay as
  default-project compat.
- cache.py: refresh_meta_repo iterates every projects row, mirroring each
  content_repo into cached_rfcs stamped with its project_id (_upsert_cached_rfc
  gains a project_id arg).
- frontend: api.listRFCs(projectId)/getRFC(projectId,slug); Catalog + RFCView
  pass useProjectId(); ProjectLayout's NotServedPlaceholder guard removed (every
  project serves), 404/not-readable branch kept; NotServedPlaceholder deleted.
- tests: test_project_scoped_serving.py (catalog scoping, entry isolation,
  gated 404); ProjectLayout.test updated (non-default renders). 445 backend +
  11 Vitest green; clean build.

Known limitation (next slice): write path + default-id re-stamp not yet scoped
(non-default project read-only); no live impact (no 2nd project in prod yet).
Per docs/superpowers/specs/2026-06-04-m3-backend-planb-design.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/components/ProjectLayout.test.jsx

@@ -43,11 +43,10 @@ describe('ProjectLayout', () => {
     expect(document.documentElement.style.getPropertyValue('--c-accent')).toBe('')
   })
 
-  it('shows the not-served placeholder for a non-default (Plan-B) project', async () => {
+  it('renders the corpus for a non-default project too (Plan B: every project serves)', async () => {
     getProject.mockResolvedValue({ id: 'other', name: 'Ecomm', type: 'bdd', theme: {} })
     renderAt('/p/other/')
-    expect(await screen.findByText(/isn.t being served here yet/i)).toBeInTheDocument()
-    expect(screen.queryByTestId('corpus')).not.toBeInTheDocument()
+    expect(await screen.findByTestId('corpus')).toBeInTheDocument()
   })
 
   it('shows a not-found message when the project 404s', async () => {