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/index.ts

@@ -1,7 +1,3 @@
-export type {
-	FeedSourceProvider,
-	FeedSourceProviderFn,
-	FeedSourceProviderInput,
-} from "./feed-source-provider.ts"
+export type { FeedSourceProvider } from "./feed-source-provider.ts"
 export { UserSession } from "./user-session.ts"
 export { UserSessionManager } from "./user-session-manager.ts"