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/api/auth_api.py

@@ -29,6 +29,8 @@ from db.db import (
     users_db, refresh_tokens_db, child_db, task_db, reward_db, image_db,
     pending_reward_db, pending_confirmations_db, tracking_events_db,
     child_overrides_db, chore_schedules_db, task_extensions_db,
+    routine_db, routine_items_db, routine_schedules_db, routine_extensions_db,
+    push_subscriptions_db, digest_action_tokens_db,
 )
 from db.default import initializeImages, createDefaultTasks, createDefaultRewards
 from api.utils import normalize_email
@@ -623,6 +625,12 @@ def e2e_seed():
     chore_schedules_db.truncate()
     task_extensions_db.truncate()
     refresh_tokens_db.truncate()
+    routine_db.truncate()
+    routine_items_db.truncate()
+    routine_schedules_db.truncate()
+    routine_extensions_db.truncate()
+    push_subscriptions_db.truncate()
+    digest_action_tokens_db.truncate()
 
     # Recreate only baseline defaults for e2e runs.
     initializeImages()