§22 M3-backend Plan B (write path, propose): project-scoped propose (v0.38.0)

A new entry can be proposed into a specific project; it lands in that project's
content repo and shows under that project's proposals. A non-default project is
no longer read-only.

- api.py: POST /api/projects/{pid}/rfcs/propose (propose body extracted into a
  project-parameterized helper; unscoped /api/rfcs/propose kept as default
  compat). Slug uniqueness, idea-PR reservation, landing state, and the
  proposed_use_cases row scoped to the target project. GET
  /api/projects/{pid}/proposals.
- cache.py: refresh_meta_pulls loops every project's content_repo, stamping
  cached_prs.project_id; projects.content_repo(pid) helper.
- frontend: proposeRFC(projectId,…)/listProposals(projectId); ProposeModal
  takes projectId; App resolves current project from the /p/<id>/ URL; Catalog
  lists that project's proposals.
- tests: test_project_scoped_propose.py (lands scoped + gated 404). 447 backend
  + 11 Vitest green; clean build.

Known limitation: branch/PR/graduation edit flows + default-id re-stamp not yet
scoped (next slice). Per docs/superpowers/specs/2026-06-04-m3-backend-planb-design.md.

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

frontend/src/api.js

@@ -198,16 +198,20 @@ export async function getRFC(projectId, slug) {
   return jsonOrThrow(await fetch(`/api/projects/${projectId}/rfcs/${slug}`))
 }
 
-export async function listProposals() {
-  return jsonOrThrow(await fetch('/api/proposals'))
+export async function listProposals(projectId) {
+  const url = projectId ? `/api/projects/${projectId}/proposals` : '/api/proposals'
+  return jsonOrThrow(await fetch(url))
 }
 
 export async function getProposal(prNumber) {
   return jsonOrThrow(await fetch(`/api/proposals/${prNumber}`))
 }
 
-export async function proposeRFC({ title, slug, pitch, tags, proposedUseCase }) {
-  const res = await fetch('/api/rfcs/propose', {
+// §22.4 (Plan B write): propose into a specific project when projectId is
+// given; else the default-project compat path.
+export async function proposeRFC(projectId, { title, slug, pitch, tags, proposedUseCase }) {
+  const url = projectId ? `/api/projects/${projectId}/rfcs/propose` : '/api/rfcs/propose'
+  const res = await fetch(url, {
     method: 'POST',
     headers: { 'Content-Type': 'application/json' },
     // #26: proposed_use_case is optional; send null when blank so the