feat(saas): add down migrations for all incremental schema changes (AUD3-DB-01)

- 16 down SQL files in migrations/down/ for each incremental migration
- db::run_down_migrations() executes rollback files in reverse order
- migrate_down CLI task: task=migrate_down timestamp=20260402
- Initial schema and seed data excluded (would be destructive)

crates/zclaw-saas/migrations/down/20260402000002_knowledge_base.sql

@@ -0,0 +1,7 @@
+-- Down: Drop knowledge base tables (reverse creation order due to FK)
+-- Note: This does NOT reverse the pgvector extension installation or role permission updates.
+DROP TABLE IF EXISTS knowledge_usage;
+DROP TABLE IF EXISTS knowledge_versions;
+DROP TABLE IF EXISTS knowledge_chunks;
+DROP TABLE IF EXISTS knowledge_items;
+DROP TABLE IF EXISTS knowledge_categories;