fix(downloads): treat recovered Patreon campaign-ID errors as non-fatal

When gallery-dl logs "[patreon][error] Failed to extract campaign ID"
but then recovers via its /cw/<vanity> fallback and skips every file
(all already archived), _categorize_error was flagging the [error] line
as fatal and returning UNKNOWN_ERROR. Add "failed to extract campaign
id" to the per-item error allowlist alongside "not allowed to view" and
"unable to get post" so skip detection proceeds and the run is
classified NO_NEW_CONTENT.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/tests/services/test_gallery_dl_categorize.py

@@ -0,0 +1,58 @@
+"""Unit tests for GalleryDLService._categorize_error.
+
+Focus: making sure errors gallery-dl logs but recovers from (e.g., Patreon's
+"Failed to extract campaign ID" that falls back to /cw/<vanity>) don't get
+misclassified as real failures when the overall run was just archive skips.
+"""
+from unittest.mock import patch
+
+from app.services.gallery_dl import ErrorType, GalleryDLService
+
+
+def _make_service():
+    """Build a service without touching the filesystem."""
+    with patch.object(GalleryDLService, "_load_base_config", return_value={}):
+        return GalleryDLService(rate_limit=1.0)
+
+
+def test_patreon_campaign_id_error_with_skips_is_no_new_content():
+    """gallery-dl logged the extractor error but recovered + skipped all → no_new_content."""
+    service = _make_service()
+    stderr = (
+        "[patreon][error] Failed to extract campaign ID\n"
+        "[patreon][info] campaign_id: 548223\n"
+        "[patreon][api] HEAD https://c10.patreonusercontent.com/4/.../img.jpg\n"
+    )
+    stdout = (
+        "# /data/downloads/ElArteDeVero/patreon/2024-01-01_1_title/01_img.jpg\n"
+        "# /data/downloads/ElArteDeVero/patreon/2024-01-02_2_title/01_img.jpg\n"
+    )
+
+    error_type, _message = service._categorize_error(1, stdout, stderr)
+
+    assert error_type == ErrorType.NO_NEW_CONTENT
+
+
+def test_patreon_campaign_id_error_without_skips_still_fails():
+    """Same error line but no skip activity → genuine failure, not masked."""
+    service = _make_service()
+    stderr = "[patreon][error] Failed to extract campaign ID\n"
+    stdout = ""
+
+    error_type, _message = service._categorize_error(1, stdout, stderr)
+
+    assert error_type != ErrorType.NO_NEW_CONTENT
+
+
+def test_patreon_campaign_id_error_alongside_real_error_still_fails():
+    """Campaign-ID error + a genuine auth error → auth error wins, not masked."""
+    service = _make_service()
+    stderr = (
+        "[patreon][error] Failed to extract campaign ID\n"
+        '[patreon][error] "GET /api/posts HTTP/1.1" 401 None\n'
+    )
+    stdout = "# /data/downloads/Foo/patreon/post1/img.jpg\n"
+
+    error_type, _message = service._categorize_error(1, stdout, stderr)
+
+    assert error_type == ErrorType.AUTH_ERROR