refactor(db): split revision preview from meta_json

Preview BIRD-фрагменты в config_revision_preview; meta_json только счётчик префиксов.

Co-authored-by: Cursor <cursoragent@cursor.com>

migrations/postgres/000015_revision_preview.down.sql

@@ -0,0 +1,6 @@
+UPDATE config_revision cr
+SET meta_json = cr.meta_json || jsonb_build_object('preview_fragments', COALESCE(p.fragments, '{}'::jsonb))
+FROM config_revision_preview p
+WHERE p.revision_id = cr.id;
+
+DROP TABLE IF EXISTS config_revision_preview;