feat: add comprehensive health check endpoint

[Adamsonoladipupo]

Summary

Added a comprehensive health check endpoint for monitoring the backend's critical dependencies.

Changes

• Added a versioned health check endpoint.
• Added MongoDB connectivity verification.
• Added Stellar Soroban RPC connectivity verification.
• Returns 200 OK when all required services are healthy.
• Returns 503 Service Unavailable when one or more services are unavailable.
• Followed the existing Controller → Service → Model architecture.
• Added robust error handling and strong TypeScript typing.
• Added tests covering healthy and unhealthy dependency scenarios.

Verification

• Unit tests: Passed
• Type checking: Passed
• Linting: Passed
• Build: Passed
• Health endpoint manually verified with a successful API response.

Proof of Work

A screenshot of the successful health endpoint response and/or passing test results is included as proof of implementation.

Closes #104

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