feat(journal): add per-user mood journal module (task 86)

Ship journal entries with mood tracking, insights, and Recharts charts.

Adds v1 API endpoints per ADR 0005 and migration 0020_journal_entries.

src/modules/journal/day-key.ts

@@ -0,0 +1,10 @@
+export function toDayKey(date: Date): string {
+  const y = date.getFullYear();
+  const m = String(date.getMonth() + 1).padStart(2, "0");
+  const d = String(date.getDate()).padStart(2, "0");
+  return `${y}-${m}-${d}`;
+}
+
+export function recordedDayKey(iso: string): string {
+  return toDayKey(new Date(iso));
+}