feat(projects): M2 — project-scoped authorization + the §22.7 resolver (§22)

Builds the three-tier authorization resolver on M1's schema spine: the
most-permissive union of deployment role (§6.1), project role (§22.6), and
per-RFC authority (§6.3/§12), with the §22.5 visibility gate subtractive on
top (§22.7). Pure app-layer — no migration (M1 shipped the tables), no
behavior change on the public default project. Verifiable on the single
default project by flipping its visibility and granting/revoking roles.

- app/auth.py: the §22.7 resolver primitives — project_visibility,
  project_member_role, project_of_rfc, is_project_superuser, can_read_project,
  effective write/discuss standing (can_contribute_in_project /
  can_discuss_in_project), require_project_readable, visible_project_ids.
  The three per-RFC capability helpers (can_discuss_rfc / can_contribute_to_rfc
  / can_invite_to_rfc) now compose all three tiers, so the ~20 call sites
  inherit M2 unchanged.
- Read pass: the §22.5 visibility gate (404 to non-members) threaded into
  every RFC-resolution helper across api.py / api_branches / api_prs /
  api_graduation / api_discussion / api_contributions / api_invitations, plus
  the catalog + proposals listings filtered by visible_project_ids.
- Write pass: the deep gates (branch read/contribute/owner, PR merge/withdraw/
  edit, graduation, discussion resolve) fold in project_admin via
  is_project_superuser; the "any-contributor" branch mode routes through
  can_contribute_in_project; propose + claim gate on project contribute
  standing. Deployment-level surfaces (admin idea-PR merge/decline, account
  notification-mute) stay deployment-scoped.
- Operator decisions: implicit-on-public (a granted deployment contributor
  keeps its pre-M2 write baseline on a public project, no membership row, so
  the N=1 case stays whole) and preserve-curation (that baseline does not
  override per-RFC owner curation — only an explicit project grant or a
  deployment owner/admin does), keeping the v0.16.0 per-RFC invite contract
  intact on public.
- tests: test_multi_project_authz_vertical.py — 9 vertical assertions on the
  resolver tiers, public-unchanged regression, the gated 404 read gate, the
  gated contribution gate, union + subtractive visibility, and revocation.
  Full suite 390 passed.
- docs: mark Part C M2 "(landed)" with the two operator decisions.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app/api_notifications.py

@@ -213,9 +213,11 @@ 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 slug FROM cached_rfcs WHERE slug = ?", (slug,)).fetchone()
+        rfc = db.conn().execute("SELECT 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.
+        auth.require_project_readable(viewer, rfc["project_id"])
         db.conn().execute(
             """
             INSERT INTO watches (user_id, rfc_slug, state, set_by, set_at, last_participation_at)