§22 S3: scope-role enforcement + collection-grain visibility (@S3) — v0.42.0

Implement slice S3 of the §22 three-tier refactor: the four-layer
most-permissive scope-role resolver (§B.2) over {owner, contributor}
grants at {global, project, collection}, with the §22.5 visibility gate
enforced at the collection grain.

- migration 030: memberships.scope_type += 'global' (the global RFC
  Contributor tier; sentinel scope_id '*').
- auth.effective_scope_role folds global → project → collection,
  most-permissive, no negative override; can_read_collection /
  can_contribute_in_collection / is_collection_superuser /
  can_create_collection gate reads, writes, admin, and create.
- collection-grain visibility: a gated collection is hidden from the
  public (404, omitted from the directory) yet visible+listed for a
  scope-role holder; a collection may be set only as strict or stricter
  than its project (public < unlisted < gated), validated at create and
  clamped at the mirror.
- entry-scoped authority (mark-reviewed, graduate, branch read/contribute,
  PR/discussion/contribution moderation) re-pointed from the project grain
  to the entry's collection.
- create-collection authority widened to a project/global-scope grant
  holder (§B.1), not only a deployment owner/admin.

Keystone reconciliation (session 0076): a plain granted account is a
granted *account*, not a write-everywhere global role; the implicit-public
write baseline is grandfathered onto the migration-seeded `default`
collection only, so the N=1 deployment loses no capability. Reinterprets
§B.1/§B.3 literally — flagged for the SPEC merge (S6).

Completes @S3 (C1.1–C1.8). Tests: test_s3_scope_roles_vertical.py (8 C.1
scenarios + visibility/strictness), test_migration_030_global_scope.py.
Full backend suite 493 passed.

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

backend/tests/test_project_scoped_propose.py

@@ -28,6 +28,13 @@ def test_propose_into_second_project_lands_scoped(app_with_fake_gitea):
     with TestClient(app) as client:
         _register_ecomm(fake)
         provision_user_row(user_id=3, login="alice", role="contributor")
+        # §22 S3: the grandfathered implicit-public baseline covers only the N=1
+        # `default` collection; a second project requires an explicit scope grant
+        # to write. Grant alice contributor at the ecomm project.
+        from app import db
+        db.conn().execute(
+            "INSERT OR REPLACE INTO memberships (scope_type, scope_id, user_id, role) "
+            "VALUES ('project', 'ecomm', 3, 'contributor')")
         sign_in_as(client, user_id=3, gitea_login="alice", display_name="Alice",
                    role="contributor", email="alice@test")
         r = client.post("/api/projects/ecomm/rfcs/propose", json={