fix(download): tier-gated = warning, race subprocess timeout, install yt-dlp

Three coupled operator-reported pains from the 2026-05-31 download
event audit:

1. `[patreon][warning] Not allowed to view post N` was bubbling up as
   an error event, bumping consecutive_failures and parking the source
   in "needs attention." The classifier's tier-gated branch was gated
   on `return_code in (1, 4)`. Gallery-dl returns a different exit
   code for mixed-failure runs (e.g. paywall warnings + a missing
   yt-dlp dep flipping the exit bits), so the branch never fired and
   the path fell through to UNKNOWN_ERROR. Widen the gate: when no
   source-level error fired AND tier-gated warnings are present,
   classify as TIER_LIMITED regardless of return code.

2. Knuxy event #38275 (2026-05-31) ran 30 min and finalized with
   "stranded by recovery sweep (no terminal status after time_limit)"
   + empty stdout/stderr. Root cause: subprocess.run timeout (900s)
   and Celery soft_time_limit (900s) raced; when Celery won, SIGKILL
   wiped the in-memory captured output and the DownloadEvent ended up
   empty-logged 18 minutes later when the sweep finalized it. Drop
   gallery-dl's default subprocess timeout to 870s — a 30s margin
   shy of Celery's soft limit — so subprocess.TimeoutExpired always
   wins the race and captures the partial stdout/stderr via the
   existing handler.

3. `[downloader.ytdl][error] Cannot import yt-dlp or youtube-dl` was
   firing on every video attachment, causing per-item download
   failures that masked legitimate tier-gated classification.
   Add yt-dlp>=2025.1 to requirements.txt. Once it's in the image,
   video posts download normally and the per-item failure noise
   disappears.

Tests added:
- pure tier-gated stderr with exit code 128 → TIER_LIMITED + success
- mixed tier-gated + yt-dlp + per-item failures → still TIER_LIMITED

requirements.txt

@@ -22,6 +22,11 @@ imagehash>=4.3,<4.4
 
 # Gallery-dl wrapper (lands in FC-3)
 gallery-dl>=1.32,<1.33
+# Video extractor backend for gallery-dl. Without it, every video post
+# attachment fails with `[downloader.ytdl][error] Cannot import yt-dlp`
+# → `[download][error] Failed to download NN_video.mp4`. Operator-flagged
+# 2026-05-31 after Patreon video posts produced empty downloads.
+yt-dlp>=2025.1
 
 # Utilities
 python-dotenv>=1.2,<2.0