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/models/source.py

@@ -26,6 +26,11 @@ class Source(Base):
 
     last_checked_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), nullable=True)
     last_error: Mapped[str | None] = mapped_column(Text, nullable=True)
+    # alembic 0032: last ErrorType category (auth_error, rate_limited,
+    # not_found, ...). Lets FailingSourcesCard surface the taxonomy as
+    # a colored chip so operators can bulk-triage by error class. Set
+    # by _update_source_health alongside last_error; cleared on 'ok'.
+    error_type: Mapped[str | None] = mapped_column(String(32), nullable=True, index=True)
     check_interval_override: Mapped[int | None] = mapped_column(Integer, nullable=True)
     consecutive_failures: Mapped[int] = mapped_column(Integer, nullable=False, default=0)