refactor: implement database backend factory for SQLite and MySQL

- Add `get_repository()` factory function to select DB implementation at runtime via DECNET_DB_TYPE env var
- Extract BaseRepository abstract interface from SQLiteRepository
- Update dependencies to use factory-based repository injection
- Add DECNET_DB_TYPE env var support (defaults to sqlite)
- Refactor models and repository base class for cross-dialect compatibility

decnet/web/db/factory.py

@@ -1,18 +1,29 @@
+"""
+Repository factory — selects a :class:`BaseRepository` implementation based on
+``DECNET_DB_TYPE`` (``sqlite`` or ``mysql``).
+"""
+from __future__ import annotations
+
+import os
 from typing import Any
-from decnet.env import os
+
 from decnet.web.db.repository import BaseRepository
 
+
 def get_repository(**kwargs: Any) -> BaseRepository:
-    """Factory function to instantiate the correct repository implementation based on environment."""
+    """Instantiate the repository implementation selected by ``DECNET_DB_TYPE``.
+
+    Keyword arguments are forwarded to the concrete implementation:
+
+    * SQLite accepts ``db_path``.
+    * MySQL accepts ``url`` and engine tuning knobs (``pool_size``, …).
+    """
     db_type = os.environ.get("DECNET_DB_TYPE", "sqlite").lower()
 
     if db_type == "sqlite":
         from decnet.web.db.sqlite.repository import SQLiteRepository
         return SQLiteRepository(**kwargs)
-    elif db_type == "mysql":
-        # Placeholder for future implementation
-        # from decnet.web.db.mysql.repository import MySQLRepository
-        # return MySQLRepository()
-        raise NotImplementedError("MySQL support is planned but not yet implemented.")
-    else:
-        raise ValueError(f"Unsupported database type: {db_type}")
+    if db_type == "mysql":
+        from decnet.web.db.mysql.repository import MySQLRepository
+        return MySQLRepository(**kwargs)
+    raise ValueError(f"Unsupported database type: {db_type}")