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/dell/run.yaml

@@ -11,7 +11,7 @@ apis:
 - tool_runtime
 - vector_io
 providers:
-  inference:
+    inference:
   - provider_id: tgi0
     provider_type: remote::tgi
     config:
@@ -93,16 +93,14 @@ providers:
     provider_type: inline::rag-runtime
 persistence:
   backends:
-    metadata_backend:
+    default:
       type: sqlite
       db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/dell}/registry.db
-    inference_backend:
-      type: sqlite
-      db_path: ${env.SQLITE_STORE_DIR:=~/.llama/distributions/dell}/inference_store.db
-  metadata:
-    backend: metadata_backend
-  inference:
-    backend: inference_backend
+  stores:
+    metadata:
+      backend: default
+    inference:
+      backend: default
 models:
 - metadata: {}
   model_id: ${env.INFERENCE_MODEL}