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/services/ytdlp.py

@@ -665,7 +665,7 @@ def predicted_file_path(video_id: str) -> Path:
     return Path(settings.download_path) / f"{video_id}.mp4"
 
 
-_SEMAPHORE = threading.Semaphore(3)
+_SEMAPHORE = threading.Semaphore(6)
 _semaphore_lock = threading.Lock()
 _cookies_browser: str = ""
 _cookies_file: str = ""
@@ -682,7 +682,7 @@ _oauth2_state_lock = threading.Lock()
 def set_max_concurrent(n: int) -> None:
     global _SEMAPHORE
     with _semaphore_lock:
-        _SEMAPHORE = threading.Semaphore(max(1, min(n, 10)))
+        _SEMAPHORE = threading.Semaphore(max(1, min(n, 16)))
 
 
 def set_cookies_browser(browser: str) -> None: