feat: update default persistence to MongoDB and enhance configuration options

README.md

@@ -38,7 +38,7 @@ npm run dev
 | `ACCOUNT_DELETION_THRESHOLD_HOURS` | Hours to wait before deleting marked accounts | 720 (30 days) |
 | `DB_ENV`                           | Database environment (`prod` or `test`)       | `prod`        |
 | `DATA_ENV`                         | Data directory environment (`prod` or `test`) | `prod`        |
-| `USE_MONGODB`                      | Use MongoDB instead of TinyDB (`true`/`false`) | `false`       |
+| `USE_MONGODB`                      | Use MongoDB (`true`/`false`) | `true`        |
 | `MONGO_URI`                        | MongoDB connection URI (required when `USE_MONGODB=true`) | —             |
 | `MONGO_DB_NAME`                    | MongoDB database name (optional)              | Parsed from `MONGO_URI`, or `chore_db`/`chore_db_test`/`chore_db_e2e` based on `DB_ENV` |
 
@@ -46,8 +46,8 @@ npm run dev
 
 The application supports two persistence backends:
 
-- **TinyDB** (default): JSON-file storage in `backend/data/db/` (or `backend/test_data/db/` for `test`/`e2e`). No extra configuration needed.
-- **MongoDB**: Set `USE_MONGODB=true` and provide `MONGO_URI`. Useful for production deployments and managed database hosting (e.g., MongoDB Atlas).
+- **MongoDB** (default): Set `MONGO_URI` (and optionally `MONGO_DB_NAME`). This is the recommended backend for production and managed hosting (e.g., MongoDB Atlas).
+- **TinyDB**: JSON-file storage in `backend/data/db/` (or `backend/test_data/db/` for `test`/`e2e`). Opt in by setting `USE_MONGODB=false`.
 
 #### Migrating from TinyDB to MongoDB
 
@@ -62,9 +62,9 @@ python -m scripts.migrate_to_mongodb
 
 The migration script reads the existing TinyDB JSON files and inserts each record into the matching MongoDB collection, skipping records that already exist. Original TinyDB files are backed up to `backend/data/db/backups/<timestamp>/`.
 
-#### Rolling Back
+#### Rolling Back to TinyDB
 
-To revert to TinyDB, simply set `USE_MONGODB=false` (or unset it). The original JSON files remain in place.
+Set `USE_MONGODB=false`. The original JSON files remain in place.
 
 #### Gunicorn / Docker