feat(backend): add PUT /api/sources/:sourceId (#87)

Add a PUT endpoint that inserts or fully replaces a user's source
config. Unlike PATCH (which deep-merges and requires an existing row),
PUT requires both `enabled` and `config`, performs an upsert via
INSERT ... ON CONFLICT DO UPDATE, and replaces config entirely.

- Add `upsertConfig` to user-sources data layer
- Add `upsertSourceConfig` to UserSessionManager
- Add `addSource` to UserSession for new source registration
- 12 new tests covering insert, replace, validation, and session refresh

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

apps/aelis-backend/src/session/user-session.ts

@@ -73,6 +73,32 @@ export class UserSession {
 		return this.sources.has(sourceId)
 	}
 
+	/**
+	 * Registers a new source in the engine and invalidates all caches.
+	 * Stops and restarts the engine to establish reactive subscriptions.
+	 */
+	addSource(source: FeedSource): void {
+		if (this.sources.has(source.id)) {
+			throw new Error(`Cannot add source "${source.id}": already registered`)
+		}
+
+		const wasStarted = this.engine.isStarted()
+
+		if (wasStarted) {
+			this.engine.stop()
+		}
+
+		this.engine.register(source)
+		this.sources.set(source.id, source)
+
+		this.invalidateEnhancement()
+		this.enhancingPromise = null
+
+		if (wasStarted) {
+			this.engine.start()
+		}
+	}
+
 	/**
 	 * Replaces a source in the engine and invalidates all caches.
 	 * Stops and restarts the engine to re-establish reactive subscriptions.