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- tests updated to the three-tier schema; test_migration_028 retired (superseded by 029)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

@@ -337,24 +337,41 @@ def project_visibility(project_id: str) -> str:
 
 
 def project_member_role(user: SessionUser | None, project_id: str) -> str | None:
-    """The user's *explicit* §22.6 project_members role in this project, or
-    None. This is the stored row only — it does not fold in the deployment tier
-    or the implicit-on-public baseline (those live in the helpers below)."""
+    """The user's *explicit* §22.6 membership role at this project, or None.
+
+    §22 three-tier (S1): M2's `project_members` rows migrated into the unified
+    `memberships` table at the project's default collection (and the project
+    tier is freshly grantable). The unified `{owner, contributor}` roles are
+    mapped back to the legacy `project_admin`/`project_contributor` strings the
+    S1 project-grain authz still speaks; the four-layer scope resolver lands in
+    S3. Reads the stored grant only (project OR default-collection scope) — it
+    does not fold in the deployment tier or the implicit-on-public baseline."""
     if user is None:
         return None
+    from . import collections as collections_mod
+    cid = collections_mod.default_collection_id(project_id)
     row = db.conn().execute(
-        "SELECT role FROM project_members WHERE project_id = ? AND user_id = ?",
-        (project_id, user.user_id),
+        "SELECT role FROM memberships "
+        "WHERE user_id = ? AND ((scope_type = 'project' AND scope_id = ?) "
+        "                    OR (scope_type = 'collection' AND scope_id = ?)) "
+        "ORDER BY CASE role WHEN 'owner' THEN 0 ELSE 1 END LIMIT 1",
+        (user.user_id, project_id, cid),
     ).fetchone()
-    return row["role"] if row else None
+    if row is None:
+        return None
+    return "project_admin" if row["role"] == "owner" else "project_contributor"
 
 
 def project_of_rfc(rfc_slug: str) -> str:
-    """The project an RFC belongs to (`cached_rfcs.project_id`). Falls back to
-    the default project when the slug isn't cached or the column is unset — the
-    same N=1 default migration 026 backfills."""
+    """The project an RFC belongs to, via its collection
+    (`cached_rfcs.collection_id` -> `collections.project_id`, §22 three-tier).
+    Falls back to the default project when the slug isn't cached — the same N=1
+    default migration 026 backfills."""
     row = db.conn().execute(
-        "SELECT project_id FROM cached_rfcs WHERE slug = ?", (rfc_slug,)
+        "SELECT c.project_id AS project_id "
+        "FROM cached_rfcs r JOIN collections c ON c.id = r.collection_id "
+        "WHERE r.slug = ?",
+        (rfc_slug,),
     ).fetchone()
     if row is None:
         return DEFAULT_PROJECT_ID