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/shuttle/__tests__/ShuttleRepositorySharedTests.test.ts

@@ -31,7 +31,7 @@ class RedisShuttleRepositoryHolder implements RepositoryHolder<ShuttleGetterSett
       url: process.env.REDIS_URL,
     });
     await this.redisClient.connect();
-    this.repo = new RedisShuttleRepository(this.redisClient);
+    this.repo = new RedisShuttleRepository(this.redisClient, 0.001, 'test-system');
     return this.repo;
   };
   teardown = async () => {