fix: show current month data and add caching (#2573)

### Summary

- Add Cache-Control headers to all route responses (1h s-maxage, 2h
stale-while-revalidate)
- Append current month to chart data so graphs stay up-to-date
(cumulative carries forward, else zero)
- Remove `.limit(12)` from growth queries for full history
- Pass isCumulative flag through addZeroMonth
- Deduplicate TransformedData type, remove transformRepoStats

apps/openpage-api/lib/growth/get-signer-conversion.ts

@@ -36,7 +36,7 @@ export const getSignerConversionMonthly = async (type: 'count' | 'cumulative' =
     ],
   };
 
-  return addZeroMonth(transformedData);
+  return addZeroMonth(transformedData, type === 'cumulative');
 };
 
 export type GetSignerConversionMonthlyResult = Awaited<