feat: per-component health probes

[AugistineCreates]

Closes #519

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Description

This PR implements the /api/admin/health/probes and /api/admin/health/probes/:component endpoints for detailed per-component health checks in the admin panel.

Requirements & Context

• Implemented GET /api/admin/health/probes to return the detailed status, response times, and error messages for all system components (api, database, and optional soroban_rpc, horizon).
• Implemented GET /api/admin/health/probes/:component to allow querying individual components directly, returning 200 if healthy/degraded, 503 if down, 404 if the component is not configured, and 400 if the component name is invalid.
• Stood up focused unit/integration tests covering all scenarios under src/routes/admin/health/probes.test.ts.
• Integrated boundary input validation using Zod schemas via the validate middleware.
• Included structured audit logging via logger.audit and correlation IDs.
• Inherited the IP allowlist and admin authentication (adminAuth) middleware from the parent /api/admin router.

Verification

Automated Tests

Added tests covering:

• Success and failure scenarios for both all-component and single-component probes.
• Boundary input validation for component name parameters.
• Verification of correct HTTP status codes (200, 400, 404, 503) under different component states.
• Mocking of external dependencies (database pool and global fetch calls).

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