Fix UI DuckDB concurrent access with read-only mode

- Add read_only parameter to DataStorage.connect()
- UI now connects in read-only mode to avoid lock conflicts with engine

src/tradefinder/data/storage.py

@@ -27,22 +27,24 @@ class DataStorage:
         storage.disconnect()
     """
 
-    def __init__(self, db_path: Path) -> None:
+    def __init__(self, db_path: Path, *, read_only: bool = False) -> None:
         """Initialize storage with database path.
 
         Args:
             db_path: Path to DuckDB database file
+            read_only: If True, open database in read-only mode (no locking)
         """
         self.db_path = db_path
+        self._read_only = read_only
         self._conn: duckdb.DuckDBPyConnection | None = None
 
     def connect(self) -> None:
         """Connect to the database."""
-        # Ensure parent directory exists
-        self.db_path.parent.mkdir(parents=True, exist_ok=True)
+        if not self._read_only:
+            self.db_path.parent.mkdir(parents=True, exist_ok=True)
 
-        self._conn = duckdb.connect(str(self.db_path))
-        logger.info("Connected to DuckDB", path=str(self.db_path))
+        self._conn = duckdb.connect(str(self.db_path), read_only=self._read_only)
+        logger.info("Connected to DuckDB", path=str(self.db_path), read_only=self._read_only)
 
     def disconnect(self) -> None:
         """Close database connection."""