feat(media): add phase5 media provider abstraction and stream credentials APIs

Backend/media/service.ts

@@ -0,0 +1,15 @@
+import { MockMediaProvider } from './providers/mock';
+import type { MediaProvider } from './types';
+
+const providerName = (process.env.MEDIA_PROVIDER ?? 'mock').toLowerCase();
+
+const createProvider = (): MediaProvider => {
+  switch (providerName) {
+    case 'mock':
+      return new MockMediaProvider();
+    default:
+      throw new Error(`Unsupported MEDIA_PROVIDER: ${providerName}`);
+  }
+};
+
+export const mediaProvider = createProvider();