Fix ruff error in OpenScale scraper

Move f-string from exception to variable assignment to comply with EM102.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

personal_data/fetchers/openscale.py

@@ -27,7 +27,8 @@ class OpenScale(Scraper):
         db_path = Path(DATABASE_PATH)
 
         if not db_path.exists():
-            raise FileNotFoundError(f'OpenScale database not found at {DATABASE_PATH}')
+            msg = f'OpenScale database not found at {DATABASE_PATH}'
+            raise FileNotFoundError(msg)
 
         with sqlite3.connect(db_path) as conn:
             conn.row_factory = sqlite3.Row