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.

alembic/versions/0032_source_error_type.py

@@ -0,0 +1,41 @@
+"""source.error_type: surface ErrorType taxonomy in FailingSourcesCard
+
+Revision ID: 0032
+Revises: 0031
+Create Date: 2026-06-02
+
+Audit 2026-06-02: the backend computes 13 ErrorType categories (auth_error,
+rate_limited, not_found, access_denied, validation_failed, etc.) and
+stamps each one on DownloadEvent.metadata, but the Source row only carried
+the free-text last_error. Operators couldn't bulk-triage failing sources
+("all auth_error → rotate cookies, all rate_limited → just wait") without
+opening Logs per row.
+
+This column receives the last error_type from _update_source_health
+and gets cleared on a successful run. Nullable + indexed so the failing-
+sources rollup can filter/group cheaply.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0032"
+down_revision: Union[str, None] = "0031"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "source",
+        sa.Column("error_type", sa.String(length=32), nullable=True),
+    )
+    op.create_index(
+        "ix_source_error_type", "source", ["error_type"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_source_error_type", table_name="source")
+    op.drop_column("source", "error_type")