feat(backend): add LocationService

- Add LocationService to manage LocationSource per user
- Add UserNotFoundError for generic user-related errors
- updateUserLocation throws if source not initialized

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

apps/aris-backend/src/lib/error.ts

@@ -0,0 +1,8 @@
+export class UserNotFoundError extends Error {
+	constructor(
+		public readonly userId: string,
+		message?: string,
+	) {
+		super(message ? `${message}: user not found: ${userId}` : `User not found: ${userId}`)
+	}
+}