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>

backend/main.py

@@ -152,7 +152,7 @@ def on_startup():
     # Backfill descriptions for videos that don't have them yet (runs in background)
     import threading
     from .routers.channels import _enrich_missing_task
-    threading.Thread(target=_enrich_missing_task, args=(10,), daemon=True).start()
+    threading.Thread(target=_enrich_missing_task, args=(3,), daemon=True).start()
 
 
 @app.get("/api/health")