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/database.py

@@ -15,6 +15,8 @@ def set_sqlite_pragma(dbapi_conn, _):
     cursor = dbapi_conn.cursor()
     cursor.execute("PRAGMA journal_mode=WAL")
     cursor.execute("PRAGMA foreign_keys=ON")
+    cursor.execute("PRAGMA busy_timeout=5000")
+    cursor.execute("PRAGMA synchronous=NORMAL")
     cursor.close()