feat(patreon): surface source URL + vanity in campaign-id resolution errors

Naming/lookup failures now report the source_url, the extracted vanity, and the
exact campaigns-API lookup URL attempted, so a "could not resolve campaign id"
error is diagnosable (wrong vanity? cookie/auth? creator renamed?) instead of
opaque. Applied to all three resolution surfaces: the native download event,
the dry-run preview, and the credential-verify probe.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/services/patreon_ingester.py

@@ -45,7 +45,7 @@ from .patreon_client import (
     PatreonDriftError,
 )
 from .patreon_downloader import PatreonDownloader
-from .patreon_resolver import resolve_campaign_id_for_source
+from .patreon_resolver import extract_vanity, resolve_campaign_id_for_source
 
 __all__ = [
     "DEAD_LETTER_THRESHOLD",
@@ -186,9 +186,11 @@ async def verify_patreon_credential(
     """
     campaign_id, _ = await resolve_campaign_id_for_source(url, cookies_path, overrides)
     if not campaign_id:
+        vanity = extract_vanity(url)
         return None, (
-            "Couldn't resolve the Patreon campaign id from the source URL — "
-            "can't verify (cookies expired, or the creator moved/renamed?)."
+            f"Couldn't resolve the Patreon campaign id — can't verify. "
+            f"source_url={url!r}; vanity={vanity!r} "
+            "(cookies expired, or the creator moved/renamed?)."
         )
     client = PatreonClient(cookies_path)
     loop = asyncio.get_running_loop()