§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>

backend/tests/test_project_scoped_serving.py

@@ -0,0 +1,67 @@
+"""§22.4 (Plan B) — per-project RFC serving. A second project's corpus renders
+under its own slug namespace via /api/projects/{pid}/rfcs[/{slug}], isolated
+from the default project and gated by §22.5 visibility."""
+from __future__ import annotations
+
+from fastapi.testclient import TestClient
+
+from test_propose_vertical import (  # noqa: F401
+    app_with_fake_gitea, tmp_env, provision_user_row, sign_in_as,
+)
+
+
+def _add_project(pid, name, vis="public"):
+    from app import db
+    db.conn().execute(
+        "INSERT OR IGNORE INTO projects (id, name, type, content_repo, visibility, initial_state) "
+        "VALUES (?, ?, 'document', ?, ?, 'super-draft')",
+        (pid, name, pid + "-content", vis),
+    )
+
+
+def _add_rfc(slug, title, pid, state="active"):
+    from app import db
+    db.conn().execute(
+        "INSERT INTO cached_rfcs (slug, title, state, project_id) VALUES (?, ?, ?, ?)",
+        (slug, title, state, pid),
+    )
+
+
+def test_catalog_scoped_to_one_project(app_with_fake_gitea):
+    app, _ = app_with_fake_gitea
+    with TestClient(app) as client:
+        _add_project("ecomm", "Ecomm")
+        _add_rfc("intro", "Default Intro", "default")
+        _add_rfc("intro", "Ecomm Intro", "ecomm")
+        _add_rfc("only-ecomm", "Ecomm Only", "ecomm")
+
+        d = client.get("/api/projects/default/rfcs").json()["items"]
+        e = client.get("/api/projects/ecomm/rfcs").json()["items"]
+        d_slugs = {i["slug"] for i in d}
+        e_slugs = {i["slug"] for i in e}
+        assert "intro" in d_slugs and "only-ecomm" not in d_slugs
+        assert {"intro", "only-ecomm"} <= e_slugs
+
+
+def test_entry_is_isolated_by_project(app_with_fake_gitea):
+    app, _ = app_with_fake_gitea
+    with TestClient(app) as client:
+        _add_project("ecomm", "Ecomm")
+        _add_rfc("intro", "Default Intro", "default")
+        _add_rfc("intro", "Ecomm Intro", "ecomm")
+        _add_rfc("only-ecomm", "Ecomm Only", "ecomm")
+
+        assert client.get("/api/projects/default/rfcs/intro").json()["title"] == "Default Intro"
+        assert client.get("/api/projects/ecomm/rfcs/intro").json()["title"] == "Ecomm Intro"
+        # a slug that exists only in ecomm 404s under default
+        assert client.get("/api/projects/ecomm/rfcs/only-ecomm").status_code == 200
+        assert client.get("/api/projects/default/rfcs/only-ecomm").status_code == 404
+
+
+def test_gated_project_catalog_404s_for_anon(app_with_fake_gitea):
+    app, _ = app_with_fake_gitea
+    with TestClient(app) as client:
+        _add_project("secret", "Secret", vis="gated")
+        _add_rfc("hush", "Hush", "secret")
+        assert client.get("/api/projects/secret/rfcs").status_code == 404
+        assert client.get("/api/projects/secret/rfcs/hush").status_code == 404