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/tfl/provider.ts

@@ -0,0 +1,19 @@
+import { TflSource, type ITflApi } from "@aris/source-tfl"
+
+import type { FeedSourceProvider } from "../session/feed-source-provider.ts"
+
+export type TflSourceProviderOptions =
+	| { apiKey: string; client?: never }
+	| { apiKey?: never; client: ITflApi }
+
+export class TflSourceProvider implements FeedSourceProvider {
+	private readonly options: TflSourceProviderOptions
+
+	constructor(options: TflSourceProviderOptions) {
+		this.options = options
+	}
+
+	feedSourceForUser(_userId: string): TflSource {
+		return new TflSource(this.options)
+	}
+}