harden: address adversarial review of Pods CRUD

Backend: atomic create with insert-retry-on-duplicate-key (removes slug
TOCTOU race + wrong 400s), resilient per-index creation so a unique-index
failure can't silently drop the constraint or block seeding, idempotent
race-safe seeding via $setOnInsert, type validation + size caps on all pod
fields/members, removeMember no-ops without a write, /api/outcome guarded.
Frontend: per-pod busy state (one mutation no longer disables every card),
joined pod derived from the list (can't go stale), create keeps inputs on error.

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

backend/src/server.ts

@@ -51,8 +51,16 @@ app.post('/api/sync-pr', async (req, res) => {
 
 // Outcome ACK -> closes the continual-learning policy loop.
 app.post('/api/outcome', async (req, res) => {
-  await recordOutcome(req.body as InterventionOutcome);
-  res.json({ ok: true });
+  try {
+    const o = (req.body ?? {}) as Partial<InterventionOutcome>;
+    if (typeof o.interventionId !== 'string' || !o.interventionId) {
+      return res.status(400).json({ error: 'interventionId is required' });
+    }
+    await recordOutcome(req.body as InterventionOutcome);
+    res.json({ ok: true });
+  } catch (e) {
+    res.status(500).json({ error: (e as Error).message });
+  }
 });
 
 // Memory counts — quick way to confirm Mongo persistence is working.