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

tests/test_gallery_dl_service.py

@@ -164,6 +164,58 @@ async def test_validation_quarantines_truncated_files(tmp_path, monkeypatch):
     assert result.error_type == ErrorType.VALIDATION_FAILED
 
 
+@pytest.mark.asyncio
+async def test_download_tier_limited_classified_as_success(gdl, monkeypatch):
+    """Pure tier-gated stderr (paywall warnings only, no source-level
+    errors). Should be TIER_LIMITED with success=True so the
+    DownloadEvent doesn't surface as 'needs attention' for an operator
+    who's just not paying the top tier. Pre-fix: this fell through to
+    UNKNOWN_ERROR when the return code wasn't 1 or 4."""
+    stderr = (
+        "[patreon][warning] Not allowed to view post 159421258\n"
+        "[patreon][warning] Not allowed to view post 159247879\n"
+        "[patreon][warning] Not allowed to view post 158111214\n"
+    )
+    # Use a non-(1,4) return code to prove the gate widened.
+    monkeypatch.setattr(
+        "backend.app.services.gallery_dl.subprocess.run",
+        lambda *a, **k: _proc(stdout="", stderr=stderr, returncode=128),
+    )
+    result = await gdl.download(
+        url="https://patreon.com/alice", artist_slug="alice", platform="patreon",
+        source_config=SourceConfig(),
+    )
+    assert result.error_type == ErrorType.TIER_LIMITED
+    assert result.success is True
+    assert "3 posts" in (result.error_message or "")
+
+
+@pytest.mark.asyncio
+async def test_download_tier_limited_with_per_item_video_failures(gdl, monkeypatch):
+    """Real operator log shape (2026-05-31): hundreds of paywall warnings
+    + a handful of per-item video errors caused by missing yt-dlp. The
+    video errors are classified as per-item (not source-level), so the
+    classifier should still pick TIER_LIMITED. Once yt-dlp is installed
+    the video errors stop firing; until then, the source-health doesn't
+    flap to 'needs attention' over paywalled content."""
+    stderr = (
+        "[patreon][warning] Not allowed to view post 159421258\n"
+        "[patreon][warning] Not allowed to view post 159247879\n"
+        "[downloader.ytdl][error] Cannot import yt-dlp or youtube-dl\n"
+        "[download][error] Failed to download 02_video.mp4\n"
+    )
+    monkeypatch.setattr(
+        "backend.app.services.gallery_dl.subprocess.run",
+        lambda *a, **k: _proc(stdout="", stderr=stderr, returncode=1),
+    )
+    result = await gdl.download(
+        url="https://patreon.com/alice", artist_slug="alice", platform="patreon",
+        source_config=SourceConfig(),
+    )
+    assert result.error_type == ErrorType.TIER_LIMITED
+    assert result.success is True
+
+
 def test_compute_run_stats(gdl):
     stats = gdl._compute_run_stats(
         return_code=0,