feat(backend): add PATCH /api/sources/:sourceId (#86)

Add endpoint for users to update their source config
and enabled state. Config is deep-merged with existing
values via lodash.merge and validated against the
provider's schema before persisting.

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

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

@@ -1,7 +1,12 @@
 import type { FeedSource } from "@aelis/core"
+import type { type } from "arktype"
+
+export type ConfigSchema = ReturnType<typeof type>
 
 export interface FeedSourceProvider {
 	/** The source ID this provider is responsible for (e.g., "aelis.location"). */
 	readonly sourceId: string
+	/** Arktype schema for validating user-provided config. Omit if the source has no config. */
+	readonly configSchema?: ConfigSchema
 	feedSourceForUser(userId: string, config: unknown): Promise<FeedSource>
 }