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":

backend/app/services/source_service.py

@@ -59,6 +59,7 @@ class SourceRecord:
     config_overrides: dict | None
     last_checked_at: str | None
     last_error: str | None
+    error_type: str | None
     check_interval_override: int | None
     consecutive_failures: int
     next_check_at: str | None
@@ -76,6 +77,7 @@ class SourceRecord:
             "config_overrides": self.config_overrides,
             "last_checked_at": self.last_checked_at,
             "last_error": self.last_error,
+            "error_type": self.error_type,
             "check_interval_override": self.check_interval_override,
             "consecutive_failures": self.consecutive_failures,
             "next_check_at": self.next_check_at,
@@ -144,6 +146,7 @@ class SourceService:
             config_overrides=source.config_overrides,
             last_checked_at=source.last_checked_at.isoformat() if source.last_checked_at else None,
             last_error=source.last_error,
+            error_type=source.error_type,
             check_interval_override=source.check_interval_override,
             consecutive_failures=source.consecutive_failures or 0,
             next_check_at=nxt.isoformat() if nxt else None,