fix(backend): use config.DB_PROVIDER instead of process.env.DB_PROVIDER in datastore.ts

- Reads validated DB_PROVIDER from Zod config instead of raw env
- Removes stale isTest fallback logic (now handled by config defaults)

backend/src/lib/datastore.ts

@@ -6,6 +6,7 @@ import {
   CosmosDatastoreProvider,
   MemoryDatastoreProvider,
 } from '@bytelyst/datastore';
+import { config } from './config.js';
 
 let provider: DatastoreProvider | null = null;
 
@@ -14,10 +15,7 @@ export function initDatastore(): DatastoreProvider {
     return provider;
   }
 
-  const isTest = process.env.NODE_ENV === 'test';
-  const dbProvider = (process.env.DB_PROVIDER || (isTest ? 'memory' : 'cosmos')).toLowerCase();
-
-  if (dbProvider === 'memory') {
+  if (config.DB_PROVIDER === 'memory') {
     provider = new MemoryDatastoreProvider();
   } else {
     provider = new CosmosDatastoreProvider();