feat(backend): add FeedEngineService

Manages FeedEngine instances per user with auto-registration of
sources from FeedSourceProvider implementations.

- Add FeedSourceProvider interface
- Add FeedEngineService with providers array injection
- Update LocationService to implement FeedSourceProvider

Co-authored-by: Ona <no-reply@ona.com>

apps/aris-backend/src/location/service.ts

@@ -1,11 +1,13 @@
 import { LocationSource, type Location } from "@aris/source-location"
 
+import type { FeedSourceProvider } from "../feed/service.ts"
+
 import { UserNotFoundError } from "../lib/error.ts"
 
 /**
  * Manages LocationSource instances per user.
  */
-export class LocationService {
+export class LocationService implements FeedSourceProvider {
 	private sources = new Map<string, LocationSource>()
 
 	/**