Adds user settings for the Castle extension

Adds functionality to configure the Castle extension, including a wallet ID.

This allows administrators to customize the extension's behavior by specifying a dedicated wallet for castle operations.

migrations.py

@@ -125,3 +125,18 @@ async def m002_add_metadata_column(db):
         ALTER TABLE entry_lines ADD COLUMN metadata TEXT DEFAULT '{}';
         """
     )
+
+
+async def m003_extension_settings(db):
+    """
+    Create extension_settings table for Castle configuration.
+    """
+    await db.execute(
+        f"""
+        CREATE TABLE extension_settings (
+            id TEXT NOT NULL PRIMARY KEY,
+            castle_wallet_id TEXT,
+            updated_at TIMESTAMP NOT NULL DEFAULT {db.timestamp_now}
+        );
+        """
+    )