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/feed-source-provider.ts

@@ -0,0 +1,9 @@
+import type { FeedSource } from "@aris/core"
+
+export interface FeedSourceProvider {
+	feedSourceForUser(userId: string): FeedSource
+}
+
+export type FeedSourceProviderFn = (userId: string) => FeedSource
+
+export type FeedSourceProviderInput = FeedSourceProvider | FeedSourceProviderFn