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/postgres/000015_revision_preview.up.sql

@@ -0,0 +1,15 @@
+-- Split BIRD preview fragments out of config_revision.meta_json (H1).
+
+CREATE TABLE config_revision_preview (
+    revision_id   UUID PRIMARY KEY REFERENCES config_revision (id) ON DELETE CASCADE,
+    fragments     JSONB NOT NULL DEFAULT '{}'
+);
+
+INSERT INTO config_revision_preview (revision_id, fragments)
+SELECT id, COALESCE(meta_json->'preview_fragments', '{}'::jsonb)
+FROM config_revision
+WHERE meta_json ? 'preview_fragments';
+
+UPDATE config_revision
+SET meta_json = meta_json - 'preview_fragments'
+WHERE meta_json ? 'preview_fragments';