feat: migrate backend persistence from TinyDB to MongoDB Atlas

- Implement lazy MongoDB client initialization in backend/db/mongo_client.py.
- Create Gunicorn configuration to ensure MongoDB client is initialized per worker.
- Refactor database access layer to support MongoDB with a new MongoLockedTable adapter.
- Add migration script to transfer existing TinyDB data to MongoDB, preserving idempotency.
- Update tracking event handling to ensure deterministic ordering with a monotonic sequence.
- Modify tests to use mongomock for MongoDB integration and ensure existing tests pass.
- Add integration test script to run tests against a local MongoDB Docker container.
- Document environment variables and migration process in specs/feat-database-migration.md.

frontend/playwright.config.ts

@@ -8,12 +8,9 @@ import {
 } from './e2e/e2e-constants'
 
 /**
- * Read environment variables from file.
- * https://github.com/motdotla/dotenv
+ * E2E MongoDB configuration is hardcoded below.  Developers can override
+ * values via environment variables; see .env.test for an example.
  */
-// import dotenv from 'dotenv';
-// import path from 'path';
-// dotenv.config({ path: path.resolve(__dirname, '.env') });
 
 /**
  * See https://playwright.dev/docs/test-configuration.
@@ -288,6 +285,8 @@ export default defineConfig({
           'BNKkHdq45uLigohSG7c1TwlAo7ETncoRVLQK02LxHgu2P1DgSJD9njRMfbbzUsaTQGllvLBz7An1WiWsNYQhvKE',
         VAPID_PRIVATE_KEY: 'jNiZJT0UO4H861KmnCt874Fg6p5jDAyYKS4V2MZf8bQ',
         PROCESS_PLATFORM: process.platform,
+        USE_MONGODB: process.env.USE_MONGODB || 'true',
+        MONGO_URI: process.env.MONGO_URI || 'mongomock',
       },
     },
   ],