migration 0006: extend generation_runs.kind to allow narrate_prep

Migration 0005 added the chapters.body_md_tts column but missed
this check constraint update — caught at runtime when
prepare-narration tried to insert kind='narrate_prep'.

Postgres doesn't ALTER CHECK in place; we drop + re-add.

migrations/0005_chapter_body_md_tts.sql

@@ -10,6 +10,3 @@
 -- whenever the author's beat-placement taste shifts.
 ALTER TABLE chapters
     ADD COLUMN body_md_tts text;
-
--- generation_runs.kind already accepts any string; no check
--- constraint to update. The new pass kind is 'narrate_prep'.