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
