§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- tests updated to the three-tier schema; test_migration_028 retired (superseded by 029)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/tests/test_registry_wiring.py

@@ -12,10 +12,14 @@ def test_startup_mirrors_registry_into_projects_and_deployment(app_with_fake_git
     app, _ = app_with_fake_gitea
     with TestClient(app):
         prow = db.conn().execute(
-            "SELECT content_repo, type, initial_state FROM projects WHERE id='default'"
+            "SELECT content_repo FROM projects WHERE id='default'"
         ).fetchone()
         assert prow["content_repo"] == "meta"   # from the registry, not META_REPO
-        assert prow["type"] == "document"
+        # §22 three-tier: type now lives on the default collection.
+        crow = db.conn().execute(
+            "SELECT type FROM collections WHERE id='default'"
+        ).fetchone()
+        assert crow["type"] == "document"
         drow = db.conn().execute("SELECT name FROM deployment WHERE id=1").fetchone()
         assert drow["name"]  # deployment name mirrored from the registry