Stop leaking internal error details on /health/deep

[agnesadoga-creator]

Closes #13

Problem

The /health/deep endpoint (and the shared health-check helper it depends on) returned raw internal error details (error.message, which can include DB connection info, internal hostnames, etc.) in the HTTP response body to any unauthenticated caller whenever a dependency check failed.

Fix

In src/health/health.service.ts, the private timed() helper (used by all dependency checks: Postgres, Redis, Horizon, Soroban RPC) no longer puts the raw caught error into the response detail field. Instead:

• The full error (including stack trace) is logged server-side via a NestJS Logger.
• The response detail is now a generic, non-sensitive string like "database check failed" — no error message or stack is exposed to the caller.

Testing

Reviewed by inspection (no local build/test run per task scope — dependencies not installed). Existing tests in test/health-deep.spec.ts and test/health-rpc.spec.ts only assert on status/latencyMs/network/timestamp fields, not on the exact detail string, so this change does not alter their expected behavior.