feat: runtime provider hotswap (#82)

Add ability to replace a FeedSourceProvider at runtime and propagate
the new source to all active (and pending) user sessions, invalidating
their feed caches.

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

apps/aelis-backend/src/session/feed-source-provider.ts

@@ -1,9 +1,7 @@
 import type { FeedSource } from "@aelis/core"
 
 export interface FeedSourceProvider {
+	/** The source ID this provider is responsible for (e.g., "aelis.location"). */
+	readonly sourceId: string
 	feedSourceForUser(userId: string): Promise<FeedSource>
 }
-
-export type FeedSourceProviderFn = (userId: string) => Promise<FeedSource>
-
-export type FeedSourceProviderInput = FeedSourceProvider | FeedSourceProviderFn