Add system ID prefix to all Redis keys to prevent cross-system ID clashes

When multiple university systems share the same Redis instance, entity IDs
(shuttles, stops, routes, etc.) could collide. This namespaces all Redis
keys with the system ID (e.g., `1:shuttle:stop:123` instead of
`shuttle:stop:123`).

- Add systemId field and prefixKey() helper to BaseRedisRepository
- Update all Redis repository subclasses to use prefixed keys
- Wire system ID from InterchangeSystem.build() into Redis repositories
- Add migration utility (migrateRedisKeysToSystemPrefix) with tests
- Update all test holders to pass a test system ID

https://claude.ai/code/session_012Vfz1NHWJbVtoDEWcE5tq6

src/repositories/notifications/RedisNotificationRepository.ts

@@ -1,3 +1,4 @@
+import { RedisClientType } from 'redis';
 import { TupleKey } from '../../types/TupleKey';
 import {
   Listener,
@@ -7,14 +8,22 @@ import {
   ScheduledNotification
 } from "./NotificationRepository";
 import { BaseRedisRepository } from "../BaseRedisRepository";
+import createRedisClientForRepository from '../../helpers/createRedisClientForRepository';
 
 export class RedisNotificationRepository extends BaseRedisRepository implements NotificationRepository {
   private notificationListeners: Listener[] = [];
   private readonly NOTIFICATION_KEY_PREFIX = 'notification:';
 
+  constructor(
+    redisClient: RedisClientType = createRedisClientForRepository(),
+    systemId: string = '',
+  ) {
+    super(redisClient, systemId);
+  }
+
   private getNotificationKey = (shuttleId: string, stopId: string): string => {
     const tuple = new TupleKey(shuttleId, stopId);
-    return `${this.NOTIFICATION_KEY_PREFIX}${tuple.toString()}`;
+    return this.prefixKey(`${this.NOTIFICATION_KEY_PREFIX}${tuple.toString()}`);
   };
 
   public addOrUpdateNotification = async (notification: ScheduledNotification): Promise<void> => {