fix(download): preserve partial output + classify timeouts richer

Operator-flagged 2026-05-30: "the fail state of timeouts doesn't show
anything other than that the task timedout and was cleaned up. I can't
tell why it ran over or if it was stuck failed or there was just that
much to get."

The TimeoutExpired branch was returning a DownloadResult with no stdout,
no stderr, no files_downloaded, and a generic "Download timed out after
N seconds" message — even though subprocess.TimeoutExpired carries the
partial output gallery-dl emitted before being killed.

Now:
- Capture e.stdout / e.stderr (coerced str if bytes; "" if None).
- Count files_downloaded from partial stdout via _count_downloaded_files.
- Surface a tail-of-stderr hint in error_message so the UI summary tells
  the operator at a glance whether it was "lots of content" (high count,
  clean stderr), "stuck retrying" (any count, 429-spam stderr), or "hung
  silent" (zero count, "no stderr output").
- Promote error_type to RATE_LIMITED when the partial stderr matches
  RATE_LIMIT_PATTERNS — gallery-dl spinning on retries through the whole
  900s window is the timeout-shaped tail of a real rate limit, and the
  platform cooldown should kick in for the same reason.

Existing test_download_timeout strengthened to also assert empty-partial
case stays correctly TIMEOUT-classified with no preserved output.
New test_download_timeout_preserves_partial_output_and_classifies covers
the rich-partial-output → RATE_LIMITED promotion path.

DownloadEvent.metadata already flows stdout/stderr/run_stats from
DownloadResult via _phase3_persist — no UI change needed; the existing
DownloadDetailModal will surface the captured output automatically once
the build redeploys.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_gallery_dl_service.py

@@ -91,6 +91,54 @@ async def test_download_timeout(gdl, monkeypatch):
     )
     assert result.success is False
     assert result.error_type == ErrorType.TIMEOUT
+    # Empty-partial timeout: no preserved output and zero files.
+    assert result.stdout == ""
+    assert result.stderr == ""
+    assert result.files_downloaded == 0
+
+
+@pytest.mark.asyncio
+async def test_download_timeout_preserves_partial_output_and_classifies(gdl, monkeypatch):
+    """When gallery-dl is killed by timeout but had emitted partial output,
+    the DownloadResult preserves stdout/stderr, counts files written so
+    far, and promotes to RATE_LIMITED if the partial stderr shows a rate-
+    limit pattern (so the platform cooldown kicks in). Without this, the
+    operator only sees 'timed out' with no clue whether it was 'lots of
+    content', 'stuck retrying', or 'hung silent'."""
+    import subprocess as sp
+
+    # Simulate gallery-dl writing two files, then spinning on 429s.
+    partial_stdout = (
+        "/tmp/images/alice/file_001.jpg\n"
+        "/tmp/images/alice/file_002.jpg\n"
+    )
+    partial_stderr = (
+        "[urllib3] 429 Too Many Requests; sleeping 60s\n"
+        "[urllib3] 429 Too Many Requests; sleeping 60s\n"
+    )
+
+    def _raise(*a, **k):
+        raise sp.TimeoutExpired(
+            cmd="gallery-dl", timeout=900,
+            output=partial_stdout, stderr=partial_stderr,
+        )
+
+    monkeypatch.setattr("backend.app.services.gallery_dl.subprocess.run", _raise)
+    result = await gdl.download(
+        url="https://patreon.com/alice", artist_slug="alice", platform="patreon",
+        source_config=SourceConfig(timeout=900),
+    )
+    assert result.success is False
+    # Partial output preserved on the DownloadResult — _phase3_persist
+    # writes them into download_event.metadata so the UI can render them.
+    assert result.stdout == partial_stdout
+    assert result.stderr == partial_stderr
+    # Files counted from partial stdout (lines starting with '/').
+    assert result.files_downloaded == 2
+    # Rate-limit pattern in partial stderr → promoted to RATE_LIMITED so
+    # _update_source_health stamps the platform cooldown.
+    assert result.error_type == ErrorType.RATE_LIMITED
+    assert "Rate-limited" in result.error_message
 
 
 @pytest.mark.asyncio