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

.vscode/launch.json

@@ -0,0 +1,33 @@
+{
+    "version": "0.2.0",
+    "configurations": [
+        {
+            "name": "Python Debugger: Flask",
+            "type": "debugpy",
+            "request": "launch",
+            "module": "flask",
+            "python": "${workspaceFolder}/.venv/bin/python",
+            "env": {
+                "FLASK_APP": "app.py",
+                "FLASK_DEBUG": "1"
+            },
+            "args": [
+                "run",
+                "--no-debugger",
+                "--no-reload",
+                "--host", "0.0.0.0",
+                "--port", "5001"
+            ],
+            "jinja": true
+        },
+        {
+            "name": "Python Debugger: Attach",
+            "type": "debugpy",
+            "request": "attach",
+            "connect": {
+                "host": "localhost",
+                "port": 5678
+            }
+        }
+    ]
+}