feat: implement caching for google sheets data

- Add cache_utils.py with JSON caching for Google Sheets
- Authenticate and cache Drive/Sheets API services globally to reuse tokens
- Use CACHE_SHEET_MAP dict to resolve cache names securely to Sheet IDs
- Change app.py data fetching to skip downloads if modifiedTime matches cache
- Replace global socket timeout with httplib2 to fix Werkzeug timeouts
- Add VS Code attach debugpy configurations to launch.json and Makefile

scripts/sync_fio_to_sheets.py

@@ -19,8 +19,14 @@ DEFAULT_SPREADSHEET_ID = "1Om0YPoDVCH5cV8BrNz5LG5eR5MMU05ypQC7UMN1xn_Y"
 SCOPES = ["https://www.googleapis.com/auth/spreadsheets"]
 TOKEN_FILE = "token.pickle"
 COLUMN_LABELS = ["Date", "Amount", "manual fix", "Person", "Purpose", "Inferred Amount", "Sender", "VS", "Message", "Bank ID", "Sync ID"]
+_SHEETS_SERVICE = None
+
 def get_sheets_service(credentials_path: str):
     """Authenticate and return the Google Sheets API service."""
+    global _SHEETS_SERVICE
+    if _SHEETS_SERVICE is not None:
+        return _SHEETS_SERVICE
+
     if not os.path.exists(credentials_path):
         raise FileNotFoundError(f"Credentials file not found: {credentials_path}")
 
@@ -50,7 +56,8 @@ def get_sheets_service(credentials_path: str):
             with open(TOKEN_FILE, "wb") as token:
                 pickle.dump(creds, token)
 
-    return build("sheets", "v4", credentials=creds)
+    _SHEETS_SERVICE = build("sheets", "v4", credentials=creds)
+    return _SHEETS_SERVICE
 
 
 def generate_sync_id(tx: dict) -> str: