feat(downloads): capture structured run stats and filtered logs at save time

Backend foundation for the upcoming Downloads modal refresh. Adds three
helpers on GalleryDLService:

  _compute_run_stats   exit_code + downloaded/skipped/per-item-failure/
                      warning counts, derived from stdout+stderr
  _extract_errors_warnings   filters stderr to just [error]/[warning] lines
                             so the modal can show a concise errors-only view
  _truncate_log        caps a log string (default 500KB) with head+tail+
                      elision marker, preventing verbose multi-hour runs
                      from bloating the download_sessions JSONB

Wires all three into download.metadata_ in the download task. No schema
change — everything nests into the existing metadata JSONB column.

Follow-up PR will consume these fields in the Downloads.vue modal.

backend/tests/services/test_gallery_dl_log_processing.py

@@ -0,0 +1,57 @@
+"""Unit tests for GalleryDLService._extract_errors_warnings and _truncate_log.
+
+These are the save-time log transforms that feed the Downloads modal's
+errors-only panel and prevent multi-MB stderr blobs from landing in JSONB.
+"""
+from app.services.gallery_dl import GalleryDLService
+
+
+def test_extract_errors_warnings_filters_debug_and_info():
+    stderr = (
+        "[gallery-dl][debug] Version 1.31.10\n"
+        "[patreon][info] campaign_id: 548223\n"
+        "[urllib3.connectionpool][debug] https://example.com 'HEAD /' 200 0\n"
+        "[download][error] Failed to download 01.part\n"
+        "[downloader.http][warning] '404 OK' for 'https://example.com'\n"
+        "[patreon][debug] skipping https://example.com/x\n"
+    )
+    result = GalleryDLService._extract_errors_warnings(stderr)
+    lines = result.splitlines()
+    assert len(lines) == 2
+    assert "[download][error]" in lines[0]
+    assert "[downloader.http][warning]" in lines[1]
+
+
+def test_extract_errors_warnings_empty_when_clean_run():
+    stderr = (
+        "[gallery-dl][debug] Version 1.31.10\n"
+        "[patreon][debug] skipping https://example.com/x\n"
+    )
+    assert GalleryDLService._extract_errors_warnings(stderr) == ""
+
+
+def test_extract_errors_warnings_handles_empty_input():
+    assert GalleryDLService._extract_errors_warnings("") == ""
+    assert GalleryDLService._extract_errors_warnings(None) == ""
+
+
+def test_truncate_log_passthrough_when_under_limit():
+    small = "line\n" * 100
+    assert GalleryDLService._truncate_log(small, max_bytes=10_000) == small
+
+
+def test_truncate_log_elides_middle_when_oversized():
+    large = ("x" * 100 + "\n") * 2000  # ~200KB
+    truncated = GalleryDLService._truncate_log(large, max_bytes=10_000)
+
+    assert len(truncated.encode("utf-8")) < len(large.encode("utf-8"))
+    assert "lines elided" in truncated
+    assert "bytes" in truncated
+    # Head and tail preserved
+    assert truncated.startswith("x")
+    assert truncated.rstrip().endswith("x")
+
+
+def test_truncate_log_handles_empty_and_none():
+    assert GalleryDLService._truncate_log("") == ""
+    assert GalleryDLService._truncate_log(None) is None

backend/tests/services/test_gallery_dl_run_stats.py

@@ -0,0 +1,81 @@
+"""Unit tests for GalleryDLService._compute_run_stats.
+
+Covers the structured counts the Downloads modal surfaces in its header row:
+exit code, downloaded count, skipped count, per-item failures, warning count.
+"""
+from unittest.mock import patch
+
+from app.services.gallery_dl import GalleryDLService
+
+
+def _make_service():
+    with patch.object(GalleryDLService, "_load_base_config", return_value={}):
+        return GalleryDLService(rate_limit=1.0)
+
+
+def test_run_stats_all_zeros_on_empty_output():
+    service = _make_service()
+    stats = service._compute_run_stats(0, "", "")
+    assert stats == {
+        "exit_code": 0,
+        "downloaded_count": 0,
+        "skipped_count": 0,
+        "per_item_failures": 0,
+        "warning_count": 0,
+    }
+
+
+def test_run_stats_counts_downloads_from_stdout_paths():
+    service = _make_service()
+    stdout = (
+        "/data/downloads/Foo/bar/1.jpg\n"
+        "/data/downloads/Foo/bar/2.jpg\n"
+        "[some log line that should not count]\n"
+    )
+    stats = service._compute_run_stats(0, stdout, "")
+    assert stats["downloaded_count"] == 2
+
+
+def test_run_stats_skips_from_both_streams():
+    """stdout '#' lines AND stderr 'skipping ' lines both count."""
+    service = _make_service()
+    stdout = (
+        "# /data/downloads/Foo/already_archived_1.jpg\n"
+        "# /data/downloads/Foo/already_archived_2.jpg\n"
+    )
+    stderr = (
+        "[patreon][debug] skipping https://example.com/a.jpg (abc image_large)\n"
+        "[patreon][debug] skipping https://example.com/b.jpg (def image_large)\n"
+        "[patreon][debug] skipping https://example.com/c.jpg (ghi image_large)\n"
+    )
+    stats = service._compute_run_stats(0, stdout, stderr)
+    assert stats["skipped_count"] == 5
+
+
+def test_run_stats_per_item_failures_counted():
+    service = _make_service()
+    stderr = (
+        "[download][error] Failed to download 02_46004845.part\n"
+        "[download][error] Failed to download 03_46004845.part\n"
+        "[patreon][debug] something unrelated\n"
+    )
+    stats = service._compute_run_stats(1, "", stderr)
+    assert stats["per_item_failures"] == 2
+
+
+def test_run_stats_warnings_counted():
+    service = _make_service()
+    stderr = (
+        "[downloader.http][warning] '404 OK' for 'https://example.com/x'\n"
+        "[patreon][warning] something else\n"
+        "[patreon][error] not a warning\n"
+    )
+    stats = service._compute_run_stats(1, "", stderr)
+    assert stats["warning_count"] == 2
+
+
+def test_run_stats_exit_code_passed_through():
+    service = _make_service()
+    assert service._compute_run_stats(0, "", "")["exit_code"] == 0
+    assert service._compute_run_stats(4, "", "")["exit_code"] == 4
+    assert service._compute_run_stats(-9, "", "")["exit_code"] == -9