[config] validate gateway mode - #277

Confidence Score: 5/5

The current PR appears safe to merge because unsupported and explicitly empty gateway modes now fail server startup instead of producing inconsistent behavior.

Summary

The PR validates gateway mode while constructing push configuration, returning a recognizable configuration error unless the mode is public or private.

• Maps public mode to FCM and private mode to upstream push explicitly.
• Adds ErrInvalidConfig for configuration error classification.
• Documents the validation behavior in a new changelog.
• Updates the environment example with existing message-hashing and OTP settings.

_{Reviews (5) · Last reviewed commit: "[docs] cleanup readme"}