chore: rework DB module (#324)

docs/reference/db.md

@@ -1,46 +1,48 @@
 # `db`
 
-Here's the reference for all database session utilities, transaction helpers, and locking functions.
+Here's the reference for the `Database` facade, the transaction helper, locking
+functions, many-to-many helpers, and row-watching utilities.
 
 You can import them directly from `fastapi_toolsets.db`:
 
 ```python
 from fastapi_toolsets.db import (
+    Database,
     LockMode,
     advisory_lock,
-    cleanup_tables,
-    create_database,
-    create_db_dependency,
-    create_db_context,
-    get_transaction,
     lock_tables,
     m2m_add,
     m2m_remove,
     m2m_set,
+    transaction,
     wait_for_row_change,
 )
 ```
 
+## ::: fastapi_toolsets.db.Database
+
+## ::: fastapi_toolsets.db.transaction
+
 ## ::: fastapi_toolsets.db.LockMode
 
-## ::: fastapi_toolsets.db.create_db_dependency
-
-## ::: fastapi_toolsets.db.create_db_context
-
-## ::: fastapi_toolsets.db.get_transaction
-
 ## ::: fastapi_toolsets.db.lock_tables
 
 ## ::: fastapi_toolsets.db.advisory_lock
 
-## ::: fastapi_toolsets.db.wait_for_row_change
-
-## ::: fastapi_toolsets.db.create_database
-
-## ::: fastapi_toolsets.db.cleanup_tables
-
 ## ::: fastapi_toolsets.db.m2m_add
 
 ## ::: fastapi_toolsets.db.m2m_remove
 
 ## ::: fastapi_toolsets.db.m2m_set
+
+## ::: fastapi_toolsets.db.wait_for_row_change
+
+Admin and test helpers live in `fastapi_toolsets.db.testing`:
+
+```python
+from fastapi_toolsets.db.testing import cleanup_tables, create_database
+```
+
+## ::: fastapi_toolsets.db.testing.create_database
+
+## ::: fastapi_toolsets.db.testing.cleanup_tables