Refactor persistence config to use stores key with unified backends

- Add StoresConfig to group all store references under persistence.stores
- Use single 'default' backend instead of separate metadata_backend/inference_backend
- Update resolver to access persistence.stores.{metadata,inference,conversations}
- All SQLite distributions now use single store.db file with shared backend

llama_stack/distributions/postgres-demo/run.yaml

@@ -80,11 +80,11 @@ persistence:
       db: ${env.POSTGRES_DB:=llamastack}
       user: ${env.POSTGRES_USER:=llamastack}
       password: ${env.POSTGRES_PASSWORD:=llamastack}
-      table_name: ${env.POSTGRES_TABLE_NAME:=llamastack_kvstore}
-  metadata:
-    backend: default
-  inference:
-    backend: default
+  stores:
+    metadata:
+      backend: default
+    inference:
+      backend: default
 models:
 - metadata: {}
   model_id: ${env.INFERENCE_MODEL}