fix: Fix delete document endpoint SQLite cursor handling

- Replace context manager pattern with direct cursor assignment
  (SQLite cursors don't support context manager protocol)
- Use conn.close() instead of db_manager.return_connection()
  (SQLiteManager doesn't have return_connection method)

Fixes 500 error when deleting documents via REST API.

mcp_server/server.py

@@ -717,20 +717,20 @@ async def api_delete_document(kb_name: str, filename: str):
         # Get the collection ID
         conn = db_manager.get_connection()
         try:
-            with conn.cursor() as cur:
+            cursor = conn.cursor()
             # Get collection ID
-                cur.execute(
+            cursor.execute(
                 "SELECT id FROM knowledge_bases WHERE name = ?",
                 (kb_name,)
             )
-                result = cur.fetchone()
+            result = cursor.fetchone()
             if not result:
                 raise HTTPException(status_code=404, detail=f"Knowledge base '{kb_name}' not found")
 
             collection_id = result[0]
 
             # Delete documents with this filename
-                cur.execute(
+            cursor.execute(
                 """
                 DELETE FROM documents
                 WHERE kb_id = ? AND json_extract(metadata, '$.filename') = ?
@@ -738,7 +738,7 @@ async def api_delete_document(kb_name: str, filename: str):
                 """,
                 (collection_id, filename)
             )
-                deleted_count = len(cur.fetchall())
+            deleted_count = len(cursor.fetchall())
             conn.commit()
 
             if deleted_count == 0:
@@ -752,7 +752,7 @@ async def api_delete_document(kb_name: str, filename: str):
                 "kb_name": kb_name
             }
         finally:
-            db_manager.return_connection(conn)
+            conn.close()
 
     except HTTPException:
         raise