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/starter/run.yaml

@@ -220,16 +220,14 @@ providers:
         db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/starter}/batches.db
 persistence:
   backends:
-    metadata_backend:
+    default:
       type: sqlite
-      db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/starter}/registry.db
-    inference_backend:
-      type: sqlite
-      db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/starter}/inference_store.db
-  metadata:
-    backend: metadata_backend
-  inference:
-    backend: inference_backend
+      db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/starter}/store.db
+  stores:
+    metadata:
+      backend: default
+    inference:
+      backend: default
 models: []
 shields:
 - shield_id: llama-guard