feat(session): query enabled sources before providers (#85)

UserSessionManager now queries the user_sources table for enabled
sources before calling any provider. Providers receive the per-user
JSON config directly instead of querying the DB themselves, removing
their db dependency and eliminating redundant round-trips.

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

apps/aelis-backend/src/sources/errors.ts

@@ -1,21 +1,3 @@
-/**
- * Thrown by a FeedSourceProvider when the source is not enabled for a user.
- *
- * UserSessionManager's Promise.allSettled handles this gracefully —
- * the source is excluded from the session without crashing.
- */
-export class SourceDisabledError extends Error {
-	readonly sourceId: string
-	readonly userId: string
-
-	constructor(sourceId: string, userId: string) {
-		super(`Source "${sourceId}" is not enabled for user "${userId}"`)
-		this.name = "SourceDisabledError"
-		this.sourceId = sourceId
-		this.userId = userId
-	}
-}
-
 /**
  * Thrown when an operation targets a user source that doesn't exist.
  */