feat: add /api/health/dependencies probes

[ValJnr-dev1]

Summary

Implements #301 — dependency health probes at /api/health/dependencies.

---

What was added

GET /api/health/dependencies

Probes all four external dependencies in parallel and returns a per-system snapshot.

HTTP Status	Meaning
200	All four probes pass (ok)
207	At least one probe is degraded, none down
503	At least one probe is down

Response shape:

{
  "status": "ok",
  "correlationId": "<uuid>",
  "checkedAt": "2026-07-24T22:00:00.000Z",
  "dependencies": {
    "postgres":     { "status": "ok", "latencyMs": 3  },
    "sorobanRpc":   { "status": "ok", "latencyMs": 12 },
    "horizon":      { "status": "ok", "latencyMs": 8  },
    "webhookQueue": { "status": "ok", "latencyMs": 1  }
  }
}

---

Files changed

File	Change
src/routes/health/dependencies.ts	New — factory createDependenciesRouter({ probeFn })
src/index.ts	Mount at /api/health/dependencies
tests/healthDependencies.test.ts	New — 25 tests
docs/health-dependencies.md	New — API runbook

---

Design notes

• Injectable probeFn — all external I/O is encapsulated in the injected function, making the route fully testable without real infrastructure (same pattern as health/ready.ts).
• Uncached — unlike /healthz/dependencies, this endpoint always runs a fresh probe. Callers that want caching can use getCachedDependencyHealth() directly.
• No auth — response contains no sensitive data. Production deployments should restrict access at the infra level (VPC/ALB).
• Structured logging — every request emits a health_dependencies_check_complete pino log line with correlationId, composite status, per-probe status, and elapsed ms.

---

Test output

PASS tests/healthDependencies.test.ts
  HTTP status codes
    ✓ returns 200 when all dependencies are ok
    ✓ returns 207 when at least one dependency is degraded (none down)
    ✓ returns 207 when all dependencies are degraded
    ✓ returns 503 when any dependency is down
    ✓ returns 503 when multiple dependencies are down
    ✓ returns 503 when a degraded + down mix exists (down wins)
  response body — status field
    ✓ body.status is 'ok' when all pass
    ✓ body.status is 'degraded' when degraded (no down)
    ✓ body.status is 'down' when any probe is down
  response shape
    ✓ includes all required top-level fields
    ✓ dependencies contains all four systems
    ✓ each dependency entry contains status and latencyMs
    ✓ checkedAt is a valid ISO-8601 timestamp
    ✓ includes per-dependency latency values from the probe
    ✓ includes error field when a probe is down
    ✓ does not expose error field when probe is ok
  correlationId
    ✓ echoes the x-correlation-id header when provided
    ✓ generates a UUID when x-correlation-id is not provided
    ✓ generates a UUID when x-correlation-id is an empty string
    ✓ correlationId differs between requests when not supplied
  authentication
    ✓ does not require an Authorization header
    ✓ ignores a supplied Authorization header (does not change the response)
  error handling
    ✓ returns 500 and propagates to errorHandler when probeFn throws
    ✓ calls probeFn exactly once per request
  default router
    ✓ exports a default dependenciesRouter as a valid Express router

Tests: 25 passed, 25 total

Lint: ✅ clean (eslint src/**/*.ts exit 0)

Closes #301

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[greatest0fallt1me]

Perfect. LGTM ✨ merging now.