feat(storage): add multi-instance storage backends per workspace

Introduce tenant-scoped storage backend registry so workspaces can register
multiple storage instances and bind each knowledge base to a specific backend,
with migration from legacy tenant-level storage config and new settings UI.

migrations/sqlite/000000_init.down.sql

@@ -21,5 +21,6 @@ DROP TABLE IF EXISTS messages;
 DROP TABLE IF EXISTS sessions;
 DROP TABLE IF EXISTS knowledges;
 DROP TABLE IF EXISTS knowledge_bases;
+DROP TABLE IF EXISTS storage_backends;
 DROP TABLE IF EXISTS models;
 DROP TABLE IF EXISTS tenants;