fix: use toISOString for trend date serialization to prevent year 2001 bug

pg driver returns PostgreSQL date as JS Date object. String(date).slice(0,10)
produced "Mon Apr 07" instead of "2026-04-07", causing chart tooltips to
default to year 2001 when parsed by new Date().

lib/queries.ts

@@ -107,7 +107,7 @@ export async function getTrends(
   );
 
   return rows.map((r) => ({
-    date: String(r.date).slice(0, 10),
+    date: r.date instanceof Date ? r.date.toISOString().slice(0, 10) : String(r.date).slice(0, 10),
     calls: Number(r.calls),
     prompt_tokens: Number(r.prompt_tokens),
     completion_tokens: Number(r.completion_tokens),