feat: warm up cache on app startup for fast first page load

Pre-fetches all 4 cached datasets (attendance, juniors, payments,
exceptions) at module load time so the first request doesn't block
on Google API calls.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app.py

@@ -55,7 +55,24 @@ def get_month_labels(sorted_months, merged_months):
             labels[m] = dt.strftime("%b %Y")
     return labels
 
+def warmup_cache():
+    """Pre-fetch all cached data so first request is fast."""
+    logger = logging.getLogger(__name__)
+    logger.info("Warming up cache...")
+    credentials_path = CREDENTIALS_PATH
+
+    get_cached_data("attendance_regular", ATTENDANCE_SHEET_ID, get_members_with_fees)
+    get_cached_data("attendance_juniors", ATTENDANCE_SHEET_ID, get_junior_members_with_fees)
+    get_cached_data("payments_transactions", PAYMENTS_SHEET_ID, fetch_sheet_data, PAYMENTS_SHEET_ID, credentials_path)
+    get_cached_data("exceptions_dict", PAYMENTS_SHEET_ID, fetch_exceptions,
+        PAYMENTS_SHEET_ID, credentials_path,
+        serialize=lambda d: [[list(k), v] for k, v in d.items()],
+        deserialize=lambda c: {tuple(k): v for k, v in c},
+    )
+    logger.info("Cache warmup complete.")
+
 app = Flask(__name__)
+warmup_cache()
 
 @app.before_request
 def start_timer():