Fix YouTube hammering, sync rate limiting, and Following load time

Sync throttling:
- sync-all now skips channels crawled within the last 6 hours (prevents
  re-scraping 1266 channels on every button press)
- Channels are queued into a single _index_channels_batch task that runs
  with 1.5s delay between each yt-dlp call instead of firing 1266
  background tasks simultaneously
- Startup enrich task reduced from 10 to 3 videos (3 yt-dlp calls on
  each container restart)
- Enrich task adds 2s sleep between metadata fetches

SQLite stability:
- busy_timeout=5000 prevents SQLITE_BUSY errors under concurrent load
- synchronous=NORMAL speeds up writes without data loss risk (safe with WAL)

Following page:
- staleTime: 60s on channels query so cached data is reused immediately
  on revisit; gcTime keeps it in memory for 5 min

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/Following.jsx

@@ -579,6 +579,8 @@ export default function Following() {
   const { data: channels = [], isLoading: loadingChannels } = useQuery({
     queryKey: ["channels"],
     queryFn: () => getChannels().then((r) => r.data),
+    staleTime: 60_000,
+    gcTime: 5 * 60_000,
   });
 
   const { data: appSettings } = useQuery({