§22 S1: thread collection_id through backend + update tests (N=1 unchanged, 454 green)

- collections.py resolution helpers (default_collection_id, type, initial_state)
- registry mirror writes project grouping fields + default-collection corpus fields
- auth.project_of_rfc joins collections; project_member_role reads memberships
- cache/api_*/funder writers+readers re-keyed to collection_id (cached_prs + denormalised
  project_id tags unchanged); api_deployment reads type/initial_state from the default collection
- projects.py restamp detects bootstrap via collections; initial_state via collection
- tests updated to the three-tier schema; test_migration_028 retired (superseded by 029)
- add @S1 acceptance test (collection grain + N=1 serving)

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

backend/app/api_notifications.py

@@ -213,7 +213,7 @@ def make_router(config: Config) -> APIRouter:
     @router.post("/api/rfcs/{slug}/watch")
     async def set_watch(slug: str, body: WatchBody, request: Request) -> dict[str, Any]:
         viewer = auth.require_user(request)
-        rfc = db.conn().execute("SELECT project_id FROM cached_rfcs WHERE slug = ?", (slug,)).fetchone()
+        rfc = db.conn().execute("SELECT (SELECT c.project_id FROM collections c WHERE c.id = cached_rfcs.collection_id) AS project_id FROM cached_rfcs WHERE slug = ?", (slug,)).fetchone()
         if rfc is None:
             raise HTTPException(404, "RFC not found")
         # §22.5 visibility gate (subtractive): gated → 404 to non-members.
@@ -222,7 +222,7 @@ def make_router(config: Config) -> APIRouter:
             """
             INSERT INTO watches (user_id, rfc_slug, state, set_by, set_at, last_participation_at)
             VALUES (?, ?, ?, 'explicit', datetime('now'), datetime('now'))
-            ON CONFLICT(project_id, user_id, rfc_slug) DO UPDATE SET
+            ON CONFLICT(collection_id, user_id, rfc_slug) DO UPDATE SET
                 state = excluded.state,
                 set_by = 'explicit',
                 set_at = excluded.set_at