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.

backend/main.py

@@ -4,7 +4,6 @@ import os
 
 from dotenv import load_dotenv
 from flask import Flask, request, jsonify
-from pymongo import MongoClient
 
 from api.admin_api import admin_api
 from api.auth_api import auth_api
@@ -28,6 +27,7 @@ from api.digest_action_api import digest_action_api
 from config.version import get_full_version
 
 from db.default import initializeImages, createDefaultTasks, createDefaultRewards
+from db.db import ensure_mongodb_indexes
 from events.broadcaster import Broadcaster
 from events.sse import sse_response_for_user, send_to_user
 from api.utils import get_current_user_id
@@ -39,6 +39,10 @@ from utils.state_expiry_scheduler import start_state_expiry_scheduler
 
 # Load environment variables
 load_dotenv()
+
+# Ensure MongoDB indexes exist when running against MongoDB.
+ensure_mongodb_indexes()
+
 # Configure logging once at application startup
 logging.basicConfig(
     level=logging.INFO,