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/server.ts

@@ -177,7 +177,11 @@ http.listen(env.PORT, '0.0.0.0', () => {
   console.log(`[server] :${env.PORT}`);
   initMemory()
     .then(() => seedDefaultPods())
-    .catch((e) => console.warn(`[memory] init failed: ${(e as Error).message}`));
+    .catch((e) => {
+      // MongoDB is mandatory — do not run a half-dead API against a broken DB.
+      console.error(`[memory] init FAILED, exiting: ${(e as Error).message}`);
+      process.exit(1);
+    });
 });
 
 let shuttingDown = false;