fix: make MongoDB mandatory, fail loud instead of degrading

Reverse the best-effort error swallowing. MongoDB is core to PodMan's
continual-learning story and must always be used, so a broken memory
layer must surface immediately rather than silently masquerade as
working (which is how observations stayed at 0 unnoticed).

- agent verifies Mongo via initMemory() at boot; bad creds / unreachable
  Atlas now fail loudly before joining the room, not mid-demo
- server exits on Mongo init failure instead of warning and limping on
- getGitStates and onScreenFrame no longer swallow Mongo errors
- memory persist() logs the failure and rethrows instead of warning

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AaCFWMkYQmTcuPsxaaACft

backend/src/memory/store.ts

@@ -10,14 +10,16 @@ import { enrichCollisionMemory } from './vectors.js';
 
 /**
  * Continual-learning memory: persist observations, collisions, interventions,
- * and outcomes to MongoDB so later sessions get sharper. Writes are best-effort
- * — a Mongo hiccup logs a warning rather than crashing the agent/server.
+ * and outcomes to MongoDB so later sessions get sharper. MongoDB is mandatory —
+ * a failed write is surfaced loudly and rethrown, never silently swallowed, so
+ * a broken memory layer can never masquerade as a working one.
  */
 async function persist(name: string, fn: () => Promise<unknown>): Promise<void> {
   try {
     await fn();
   } catch (err) {
-    console.warn(`[memory] ${name} persist failed: ${(err as Error).message}`);
+    console.error(`[memory] ${name} persist FAILED: ${(err as Error).message}`);
+    throw err;
   }
 }