refactor: consolidate backend services

Replace per-source services (LocationService, WeatherService,
TflService, FeedEngineService) with a single UserSessionManager
that owns all per-user state. Source creation is delegated to
thin FeedSourceProvider implementations per source type.

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

apps/aris-backend/src/session/index.ts

@@ -0,0 +1,7 @@
+export type {
+	FeedSourceProvider,
+	FeedSourceProviderFn,
+	FeedSourceProviderInput,
+} from "./feed-source-provider.ts"
+export { UserSession } from "./user-session.ts"
+export { UserSessionManager } from "./user-session-manager.ts"