Implement Phases 1-5: skeleton, OAuth, categories, video sync/feed, playback

- FastAPI + PostgreSQL + Alembic + React/Vite skeleton, Docker Compose, healthcheck
- Google OAuth (single allowed account), encrypted refresh token storage
- Subscriptions sync with pagination, uploads playlist batch fetch
- Categories CRUD, many-to-many channel assignment, category filtering
- Video sync (playlistItems + videos.list batching), cached feed with cursor
  pagination, background scheduler (APScheduler)
- Video detail page with YouTube embed player
- SPA fallback routing, optimistic UI updates, client-side query caching

40 backend tests covering OAuth allow-list, sync idempotency, cascade deletes,
cursor pagination, and category filtering.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/utils/format.ts

@@ -0,0 +1,35 @@
+export function formatDuration(seconds: number | null): string | null {
+  if (seconds == null) return null
+  const h = Math.floor(seconds / 3600)
+  const m = Math.floor((seconds % 3600) / 60)
+  const s = Math.floor(seconds % 60)
+  if (h > 0) {
+    return `${h}:${String(m).padStart(2, '0')}:${String(s).padStart(2, '0')}`
+  }
+  return `${m}:${String(s).padStart(2, '0')}`
+}
+
+const RTF = new Intl.RelativeTimeFormat('ru', { numeric: 'auto' })
+
+export function formatRelativeTime(iso: string): string {
+  const date = new Date(iso)
+  const diffMs = date.getTime() - Date.now()
+  const diffSeconds = Math.round(diffMs / 1000)
+  const absSeconds = Math.abs(diffSeconds)
+
+  const units: [Intl.RelativeTimeFormatUnit, number][] = [
+    ['year', 365 * 24 * 3600],
+    ['month', 30 * 24 * 3600],
+    ['week', 7 * 24 * 3600],
+    ['day', 24 * 3600],
+    ['hour', 3600],
+    ['minute', 60],
+  ]
+
+  for (const [unit, secondsInUnit] of units) {
+    if (absSeconds >= secondsInUnit) {
+      return RTF.format(Math.round(diffSeconds / secondsInUnit), unit)
+    }
+  }
+  return RTF.format(diffSeconds, 'second')
+}