feat(api): user-scoped CSV export matching legacy format

apps/api/src/lib/csv.ts

@@ -0,0 +1,13 @@
+// CSV helpers ported verbatim from the legacy export route (docs/reference/legacy-backend.md §4).
+
+// Wrap a value in double quotes, doubling any embedded double quote (standard CSV escaping).
+export const quote = (value: unknown) => `"${String(value).replace(/"/g, '""')}"`;
+
+// Format a total number of seconds as zero-padded HH:MM:SS (hours can exceed 99).
+export function formatDuration(totalSeconds: number): string {
+  const s = totalSeconds || 0;
+  const h = Math.floor(s / 3600);
+  const m = Math.floor((s % 3600) / 60);
+  const sec = s % 60;
+  return `${String(h).padStart(2, '0')}:${String(m).padStart(2, '0')}:${String(sec).padStart(2, '0')}`;
+}