Fix popular fetch and improve date/view_count coverage

Popular fetch now does a two-phase approach: fast flat-playlist to get
IDs in popularity order, then parallel full metadata fetch (8 workers)
to get real view_count and published_at for each video. Previously
flat-playlist mode returned timestamp/view_count as null.

Enrich task now also backfills published_at and view_count (not just
description). Startup limit 3→50, enrichment sleep 2s→0.5s.

Raise all thread pool sizes to match 8-core machine:
- Discovery search: 5→8 workers
- Graph signal: 4→8 workers
- Popular fetch: 5→8 workers
- Download semaphore default 3→6, cap 10→16

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

backend/main.py

@@ -168,7 +168,7 @@ 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, _index_channels_batch
-    threading.Thread(target=_enrich_missing_task, args=(3,), daemon=True).start()
+    threading.Thread(target=_enrich_missing_task, args=(50,), daemon=True).start()
 
     def _auto_sync_daemon():
         import time