Implement city name validation

[shishnk]

Currently, the system allows users to subscribe to any city name without proper validation. To improve data integrity and prevent invalid city subscriptions, implement city name validation. The validation process should involve storing a list of valid city names locally and performing case-insensitive checks against this list when users subscribe to new cities. If the provided city name is not found in the list, reject the subscription request and notify the user of the invalid city name.