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/app/services/gallery_dl.py

@@ -434,12 +434,19 @@ class GalleryDLService:
         ]
         has_actual_error = any(err in combined for err in actual_error_indicators)
 
-        # Per-item access warnings (e.g., "Not allowed to view post" on Patreon
-        # /c/user/posts URLs) are normal in multi-post downloads where some posts
-        # are paywalled. These shouldn't block skip detection when the overall
-        # download proceeded successfully with skips.
+        # Non-fatal error lines gallery-dl logs but recovers from. These shouldn't
+        # block skip detection when the overall download proceeded successfully:
+        #   - "not allowed to view" / "unable to get post": per-item paywall warnings
+        #     on Patreon /c/user/posts URLs where some posts are paywalled.
+        #   - "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.
         if has_actual_error and (skip_line_count > 0 or has_skip_text):
-            per_item_patterns = ["not allowed to view", "unable to get post"]
+            per_item_patterns = [
+                "not allowed to view",
+                "unable to get post",
+                "failed to extract campaign id",
+            ]
             error_lines = [
                 line for line in combined.split('\n')
                 if any(ind in line for ind in actual_error_indicators)