refactor(db): split revision preview from meta_json

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

Co-authored-by: Cursor <[email protected]>

migrations/sqlite/000015_revision_preview.down.sql

@@ -0,0 +1,9 @@
+UPDATE config_revision
+SET meta_json = json_set(
+    meta_json,
+    '$.preview_fragments',
+    json(COALESCE((SELECT fragments FROM config_revision_preview p WHERE p.revision_id = config_revision.id), '{}'))
+)
+WHERE id IN (SELECT revision_id FROM config_revision_preview);
+
+DROP TABLE IF EXISTS config_revision_preview;