feat(backend): persist memory to MongoDB

Replace the in-memory store with a real MongoDB-backed store. Adds memory/db.ts
(client singleton, typed collections, startup ping + indexes) and rewrites
record{Observation,Collision,Intervention,Outcome} to insert into Mongo
(best-effort — failures log, don't crash). Server connects on startup and
exposes GET /api/memory/stats for verification.

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

backend/src/memory/db.ts

@@ -0,0 +1,51 @@
+import { MongoClient, type Db, type Collection } from 'mongodb';
+import type { EngineerContext, Collision, Intervention, InterventionOutcome } from '@podman/shared';
+import { env } from '../env.js';
+
+let clientPromise: Promise<MongoClient> | null = null;
+
+function getClient(): Promise<MongoClient> {
+  if (!clientPromise) {
+    const client = new MongoClient(env.MONGODB_URI, { serverSelectionTimeoutMS: 3000 });
+    clientPromise = client.connect();
+  }
+  return clientPromise;
+}
+
+/** The PodMan database (name comes from the MONGODB_URI path, e.g. `podman`). */
+export async function getDb(): Promise<Db> {
+  const client = await getClient();
+  return client.db();
+}
+
+export interface PodCollections {
+  observations: Collection<EngineerContext>;
+  collisions: Collection<Collision>;
+  interventions: Collection<Intervention>;
+  outcomes: Collection<InterventionOutcome>;
+}
+
+export async function collections(): Promise<PodCollections> {
+  const db = await getDb();
+  return {
+    observations: db.collection<EngineerContext>('observations'),
+    collisions: db.collection<Collision>('collisions'),
+    interventions: db.collection<Intervention>('interventions'),
+    outcomes: db.collection<InterventionOutcome>('outcomes'),
+  };
+}
+
+/** Connect, verify reachability, and create helpful indexes. Call once on startup. */
+export async function initMemory(): Promise<void> {
+  const db = await getDb();
+  await db.command({ ping: 1 });
+  const c = await collections();
+  await Promise.all([
+    c.observations.createIndex({ podId: 1, observedAt: -1 }),
+    c.observations.createIndex({ engineerId: 1 }),
+    c.collisions.createIndex({ podId: 1, detectedAt: -1 }),
+    c.interventions.createIndex({ collisionId: 1 }),
+    c.outcomes.createIndex({ interventionId: 1 }),
+  ]);
+  console.log(`[memory] mongo connected -> ${db.databaseName}`);
+}