§22 S6: request-to-join + cross-collection inbox (§22.8) — v0.46.0

Ships the request side of joining a gated scope, completing the §22.8 pair
(S4 shipped the invite half). A user who knows a project/collection exists
asks to join it naming a desired role; the request fans out to that scope's
Owners across the subtree (the cross-collection inbox, §22.11), who accept
(writing the memberships row via memberships.grant) or decline. Built by
analogy to §28 contribution_requests + the S4 memberships surface.

Backend
- migration 032: join_requests (scope_type ∈ {project,collection}, scope_id,
  requester, requested_role, message, status, granted_role); one-open-per
  (scope, requester) partial unique index. Additive — no rebuild.
- api_join_requests.py: GET join-target / POST join-requests / POST
  {id}/accept / {id}/decline under /api/scopes/{scope_type}/{scope_id}/.
  Accept grants via memberships.grant; the request POST does not require the
  scope be readable (that is how one joins a gated scope).
- notify: fan_out_join_request (subtree-Owner enumeration via
  _scope_owner_user_ids), notify_join_decided, 3 render_summary cases.
- auth.effective_role_at_scope — scope-grain twin of effective_scope_role,
  folding global → project for a project target.
- api_collections: viewer.can_request_join on the project + collection blocks.

Frontend
- api.js join verbs; JoinRequestModal; "Request to join" affordance in the
  collection directory + catalog footer; JoinRequestRow in the inbox.

Tests: backend test_join_requests_vertical (11) + test_migration_032 (5);
frontend api.joinrequests + CollectionDirectory cases. 546 backend / 36
frontend green.

Per docs/design/2026-06-05-three-tier-projects-collections.md Part E (S6) and
SPEC.md §22.8 / §22.11. Closes the request-to-join item flagged open at
0.45.0; per-type surfaces (§22.4a items 1 & 3) remain the last S6 item.

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

frontend/src/components/CollectionDirectory.jsx

@@ -20,6 +20,7 @@ import { collectionHome } from '../lib/entryPaths'
 import { entryNoun } from './ProjectLayout.jsx'
 import CreateCollectionModal from './CreateCollectionModal.jsx'
 import ScopeMembersModal from './ScopeMembersModal.jsx'
+import JoinRequestModal from './JoinRequestModal.jsx'
 
 export default function CollectionDirectory({ projectId }) {
   const [cols, setCols] = useState(null)
@@ -27,6 +28,7 @@ export default function CollectionDirectory({ projectId }) {
   const [version, setVersion] = useState(0)
   const [createOpen, setCreateOpen] = useState(false)
   const [membersOpen, setMembersOpen] = useState(false)
+  const [joinOpen, setJoinOpen] = useState(false)
 
   useEffect(() => {
     let live = true
@@ -46,6 +48,7 @@ export default function CollectionDirectory({ projectId }) {
 
   const canCreate = !!viewer?.can_create_collection
   const canInvite = !!viewer?.can_invite
+  const canRequestJoin = !!viewer?.can_request_join
 
   return (
     <main className="chrome-pane">
@@ -56,6 +59,9 @@ export default function CollectionDirectory({ projectId }) {
             {canInvite && (
               <button className="btn-link" onClick={() => setMembersOpen(true)}>Members</button>
             )}
+            {canRequestJoin && (
+              <button className="btn-link" onClick={() => setJoinOpen(true)}>Request to join</button>
+            )}
             {canCreate && cols.length > 0 && (
               <button className="btn-primary" onClick={() => setCreateOpen(true)}>New collection</button>
             )}
@@ -101,6 +107,13 @@ export default function CollectionDirectory({ projectId }) {
           onClose={() => setMembersOpen(false)}
         />
       )}
+      {joinOpen && (
+        <JoinRequestModal
+          scopeType="project"
+          scopeId={projectId}
+          onClose={() => setJoinOpen(false)}
+        />
+      )}
     </main>
   )
 }