feat(backend): : improve backend reliability, observability, and even…

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Closes #1161
Closes #1167
Closes #1170
Closes #1171

feat(backend): improve backend reliability, observability, and event processing

Summary

This PR introduces several backend improvements focused on application reliability, monitoring, and event processing. The changes improve server lifecycle management, expose runtime health information, prevent duplicate event processing, and add transaction fee statistics logging from Stellar Horizon.

Changes

Graceful shutdown handling

• Implement graceful shutdown signal handling in main.rs.
• Ensure the Axum server exits cleanly on termination signals.
• Gracefully stop background workers and release application resources.
• Prevent abrupt shutdowns that could interrupt in-flight requests or background tasks.

Health dashboard API

• Add a health dashboard endpoint for backend monitoring.
• Expose service health and runtime status.
• Follow existing Axum routing conventions under routes/v1.rs.
• Provide a structured response suitable for dashboards and monitoring tools.

Duplicate event protection

• Prevent duplicate processing within the event listener.
• Add safeguards to ensure the same event is processed only once.
• Improve event synchronization reliability and overall system consistency.

Stellar Horizon fee statistics

• Log transaction fee statistics retrieved from Stellar Horizon.
• Improve backend observability with additional runtime metrics.
• Make fee information available for operational monitoring and debugging.

Testing

Verified by running the backend test suite:

cd backend
cargo test

Impact

• Improved application shutdown reliability.
• Better operational visibility through health monitoring.
• More resilient event processing by eliminating duplicate handling.
• Enhanced logging and observability for Stellar network transaction fees.

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[martinvibes]

Nice implemention
keep it up