fix(generation): repair articles stuck in generating when latest task is terminal

The state-consistency worker now detects articles still flagged as
generating whose latest generation_task is already failed or completed
(with a version) and aligns the article status to the terminal task,
invalidating article caches on the way out. Adds supporting partial
indexes and switches UpdateGenerationTaskStatus to COALESCE started_at
and completed_at so partial status updates don't wipe the timestamps.

server/internal/tenant/repository/generated/audit.sql.go

@@ -125,8 +125,8 @@ func (q *Queries) CreateTaskRecord(ctx context.Context, arg CreateTaskRecordPara
 
 const updateGenerationTaskStatus = `-- name: UpdateGenerationTaskStatus :exec
 UPDATE generation_tasks SET status = $1::text, error_message = $2,
-       started_at = $3,
-       completed_at = $4,
+       started_at = COALESCE($3, started_at),
+       completed_at = COALESCE($4, completed_at),
        lease_token = CASE WHEN $1::text IN ('completed', 'failed') THEN NULL ELSE lease_token END,
        lease_owner = CASE WHEN $1::text IN ('completed', 'failed') THEN NULL ELSE lease_owner END,
        lease_expires_at = CASE WHEN $1::text IN ('completed', 'failed') THEN NULL ELSE lease_expires_at END,