fix(audit-g5d): surface ErrorType taxonomy on FailingSourcesCard

Alembic 0032 adds Source.error_type (varchar(32), indexed).
_update_source_health stamps it alongside last_error on status='error'
and clears it on 'ok'. SourceRecord/to_dict exposes it.

FailingSourcesCard renders a colored chip next to the consecutive-
failures count, with a tooltip explaining the suggested operator
action. Color reflects intent:
  - warning (yellow) — operator action needed (auth_error)
  - info (blue)      — backend-paced (rate_limited / timeout /
                       network_error / partial / tier_limited)
  - error (red)      — likely terminal without intervention
                       (not_found / access_denied / validation_failed /
                        unsupported_url / http_error / unknown_error)

Audit 2026-06-02: the backend computed 13 ErrorType categories but
only the free-text last_error reached the operator. Bulk-triage by
class ("all auth_error → rotate cookies", "12 rate_limited → just
wait") required opening Logs per row.

backend/app/services/download_service.py

@@ -434,9 +434,15 @@ class DownloadService:
         if status == "ok":
             source.consecutive_failures = 0
             source.last_error = None
+            # alembic 0032 — clear the failure-class chip on success.
+            source.error_type = None
         elif status == "error":
             source.consecutive_failures = (source.consecutive_failures or 0) + 1
             source.last_error = error_message
+            # alembic 0032 — stamp the failure-class so FailingSourcesCard
+            # can render a colored chip and operators can bulk-triage
+            # by error class without opening Logs per row.
+            source.error_type = error_type
             if error_type == "rate_limited":
                 await set_platform_cooldown(self.async_session, source.platform)
         elif status == "skipped":