fix(downloads): don't classify per-item download 404s as source-level Not Found

gallery-dl logs `[download][error] Failed to download <file>.part` when a
single post attachment's media URL expires or is deleted, then recovers
and proceeds with the rest of the run. Our error categorizer was letting
a urllib3 debug line like `"HEAD /media-u/v3/<id> HTTP/1.1" 404 0` match
the NOT_FOUND_PATTERNS list, flagging the whole source as deleted even
though gallery-dl kept downloading subsequent items successfully.

Adds "failed to download" to the per-item allowlist so the mask flips
when skip activity is present, matching the shape of the campaign-ID
fix. Total-failure runs still classify as failures.

Also renames the Download Details modal's "Error Log" panel to
"Verbose Log (stderr)" — gallery-dl's `-v` flag sends all logging
(including [debug] lines) to stderr, so the original label was
misleading. Drops the blanket red text color for the same reason.

backend/app/services/gallery_dl.py

@@ -441,11 +441,15 @@ class GalleryDLService:
         #   - "failed to extract campaign id": logged when the vanity-URL HTML path
         #     fails, but gallery-dl falls back to the /cw/<vanity> redirect and
         #     recovers the campaign_id on its own.
+        #   - "failed to download": per-item download failure (e.g., a single
+        #     Patreon attachment whose media URL expired / was deleted). The
+        #     downloader logs [download][error] and moves to the next item.
         if has_actual_error and (skip_line_count > 0 or has_skip_text):
             per_item_patterns = [
                 "not allowed to view",
                 "unable to get post",
                 "failed to extract campaign id",
+                "failed to download",
             ]
             error_lines = [
                 line for line in combined.split('\n')

backend/tests/services/test_gallery_dl_categorize.py

@@ -56,3 +56,40 @@ def test_patreon_campaign_id_error_alongside_real_error_still_fails():
     error_type, _message = service._categorize_error(1, stdout, stderr)
 
     assert error_type == ErrorType.AUTH_ERROR
+
+
+def test_patreon_per_item_download_404_with_skips_is_no_new_content():
+    """Per-item 'Failed to download' + urllib3 404 HEAD + skips → no_new_content.
+
+    Real-world trigger: a Patreon post's media URL (e.g., /media-u/v3/<id>)
+    expired or was deleted. gallery-dl logs [download][error] and moves on;
+    other items skip cleanly. We shouldn't classify the whole source NOT_FOUND
+    just because one attachment 404'd.
+    """
+    service = _make_service()
+    stderr = (
+        "[download][error] Failed to download 02_46004845.part\n"
+        '[urllib3.connectionpool][debug] "HEAD /media-u/v3/46004845 HTTP/1.1" 404 0\n'
+        "[downloader.http][warning] '404 OK' for 'https://www.patreon.com/media-u/v3/46004845'\n"
+        "[download][error] Failed to download 03_46004845.part\n"
+        "[patreon][debug] skipping https://c10.patreonusercontent.com/4/patreon-media/p/post/30953341/.../1.png (abc image_large)\n"
+    )
+    stdout = ""
+
+    error_type, _message = service._categorize_error(1, stdout, stderr)
+
+    assert error_type == ErrorType.NO_NEW_CONTENT
+
+
+def test_patreon_per_item_download_failure_without_skips_still_fails():
+    """Same 'Failed to download' error but no skip activity → real failure."""
+    service = _make_service()
+    stderr = (
+        "[download][error] Failed to download 02_46004845.part\n"
+        '[urllib3.connectionpool][debug] "HEAD /media-u/v3/46004845 HTTP/1.1" 404 0\n'
+    )
+    stdout = ""
+
+    error_type, _message = service._categorize_error(1, stdout, stderr)
+
+    assert error_type != ErrorType.NO_NEW_CONTENT

frontend/src/views/Downloads.vue

@@ -334,9 +334,9 @@
               </v-expansion-panel-text>
             </v-expansion-panel>
             <v-expansion-panel v-if="selectedDownload.metadata.stderr">
-              <v-expansion-panel-title>Error Log</v-expansion-panel-title>
+              <v-expansion-panel-title>Verbose Log (stderr)</v-expansion-panel-title>
               <v-expansion-panel-text>
-                <pre class="text-caption text-error" style="white-space: pre-wrap; max-height: 300px; overflow: auto">{{ selectedDownload.metadata.stderr }}</pre>
+                <pre class="text-caption" style="white-space: pre-wrap; max-height: 300px; overflow: auto">{{ selectedDownload.metadata.stderr }}</pre>
               </v-expansion-panel-text>
             </v-expansion-panel>
           </v-expansion-panels>