chore: rework DB module (#324)

src/fastapi_toolsets/db/__init__.py

@@ -0,0 +1,18 @@
+"""Database package: the ``Database`` facade plus PostgreSQL power-tools."""
+
+from .core import Database, transaction
+from .locks import LockMode, advisory_lock, lock_tables
+from .m2m import m2m_add, m2m_remove, m2m_set
+from .watch import wait_for_row_change
+
+__all__ = [
+    "Database",
+    "LockMode",
+    "advisory_lock",
+    "lock_tables",
+    "m2m_add",
+    "m2m_remove",
+    "m2m_set",
+    "transaction",
+    "wait_for_row_change",
+]