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/tests/conftest.py

@@ -1,5 +1,7 @@
 import os
 os.environ['DB_ENV'] = 'test'
+os.environ['USE_MONGODB'] = 'true'
+os.environ['MONGO_URI'] = 'mongomock'
 os.environ.setdefault('SECRET_KEY', 'test-secret-key')
 os.environ.setdefault('REFRESH_TOKEN_EXPIRY_DAYS', '90')
 os.environ.setdefault('DIGEST_TOKEN_SECRET', 'test-digest-secret')
@@ -18,5 +20,11 @@ TEST_REFRESH_TOKEN_EXPIRY_DAYS = 90
 @pytest.fixture(scope="session", autouse=True)
 def set_test_db_env():
     os.environ['DB_ENV'] = 'test'
+    os.environ['USE_MONGODB'] = 'true'
+    os.environ['MONGO_URI'] = 'mongomock'
     os.environ['SECRET_KEY'] = TEST_SECRET_KEY
-    os.environ['REFRESH_TOKEN_EXPIRY_DAYS'] = str(TEST_REFRESH_TOKEN_EXPIRY_DAYS)
+    os.environ['REFRESH_TOKEN_EXPIRY_DAYS'] = str(TEST_REFRESH_TOKEN_EXPIRY_DAYS)
+    # Ensure indexes are created once for the test session.  This is safe to
+    # call repeatedly because MongoDB treats index creation as idempotent.
+    from db.db import ensure_mongodb_indexes
+    ensure_mongodb_indexes()