feat: reduce TTL and timeouts for debugging

[takaokouji]

Summary

Reduced TTL and timeouts to facilitate debugging.

Changes

• Reduced Group TTL from 5 minutes to 1 minute.
• Reduced Group expiration from 90 minutes to 50 minutes.
• Updated checkGroupExists and renewHeartbeat to use new thresholds.
• Updated heartbeat_spec.rb to verify 50-minute expiration.

Testing

• Ran RSpec request specs against staging environment.
• Verified that expiresAt is correctly calculated (~50 mins).
• Validated E2E subscriptions.