API rollout gating: return 503 for unavailable event-based endpoints

[ylembachar]

The API supports two endpoint families: time-based endpoints and event-based endpoints. These are not rolled out uniformly across networks yet. Time-based endpoints are available, while event-based endpoints may be unavailable depending on network/configuration.

Proposed solution:

• Keep time endpoints available as intended
• Disable event-based endpoints if no event registration contract is defined in the config
• Return a “Service Unavailable” style response (HTTP 503) for event-based endpoints
• Update README Endpoints Important Note to reflect this behavior

This gives users immediate, explicit feedback about rollout status and reduces confusion during integration.

[blockchainluffy]

In my opinion 501 error code best suites here, as it suggests that a feature is not available, but 503 implies that Service Unavailable indicating that a server is temporarily unable to handle a request, usually due to overloading, scheduled maintenance, or service downtime