feat(backend): make FeedSourceProvider async (#77)

* feat(backend): make FeedSourceProvider async

Make feedSourceForUser and FeedSourceProviderFn return promises.
Use Promise.allSettled to tolerate partial provider failures.
Guard concurrent getOrCreate calls with in-flight promise dedup.
Return 503 from HTTP handlers when session creation fails.

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

* fix(backend): handle remove() during in-flight session creation

Cancel pending getOrCreate when remove() is called mid-flight.
Destroy the resulting session to prevent it from leaking.

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

---------

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

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

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