refactor(db): mark prefix snapshot contract migration

Маркер миграции contract-фазы H2.

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

migrations/postgres/000018_prefix_snapshot_contract.down.sql

@@ -0,0 +1 @@
+COMMENT ON COLUMN config_revision.prefix_snapshot_id IS NULL;

migrations/postgres/000018_prefix_snapshot_contract.up.sql

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+-- H2 contract marker: new revisions use prefix_snapshot_id only (enforced in application code).
+
+COMMENT ON COLUMN config_revision.prefix_snapshot_id IS 'Materialized prefixes; revision_materialized_prefix deprecated for new rows';