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/migrations/20260511110000_generation_task_terminal_repair_indexes.down.sql

@@ -0,0 +1,3 @@
+DROP INDEX IF EXISTS idx_articles_generating_state_repair;
+DROP INDEX IF EXISTS idx_generation_tasks_article_latest_state_repair;
+DROP INDEX IF EXISTS idx_generation_tasks_recent_terminal_state_repair;