[Quality-2] Make readiness probes reflect required dependency classes

[Gwther]

Overview

This PR updates the deep readiness probe so /api/health/ready reflects all required runtime dependency classes. The endpoint now reports ready only when Postgres, Soroban RPC, indexer lag, Redis/BullMQ, and Horizon are all healthy.

Related Issue

Closes #939

Changes

Readiness dependency coverage

• [MODIFY] src/services/readinessService.ts

  • Adds a Horizon readiness probe using the configured HORIZON_URL.
  • Treats non-2xx Horizon responses, network failures, and 1-second timeout aborts as readiness failures.
  • Includes Horizon in the aggregate Promise.allSettled readiness result.

• [MODIFY] src/routes/health/ready.ts

  • Updates endpoint documentation to list all five required readiness checks.

• [MODIFY] tests/healthReady.test.ts

  • Adds Horizon success, HTTP failure, network failure, and timeout coverage.
  • Verifies the HTTP response includes all five required checks.
  • Verifies a Horizon failure produces a 503 unready response.
  • Keeps aggregate readiness tests isolated from real Stellar network calls.

• [MODIFY] README.md, docs/health-ready.md

  • Updates readiness examples, dependency descriptions, environment-variable docs, and logging examples for the Horizon check.

Verification Results

git diff --check origin/main...HEAD
PASS

npm test -- tests/healthReady.test.ts --runInBand
BLOCKED: local dependencies are not installed; `jest` is not available.

npm ci
BLOCKED: local C: drive has insufficient free space for dependency installation.

Acceptance Criteria

Acceptance Criteria	Status
Deterministic valid, invalid, duplicate, and boundary behavior	Addressed: Horizon pass, HTTP failure, network failure, timeout boundary, and aggregate unready behavior are covered in focused tests.
Authorization, validation, and state-transition invariants remain enforced	Addressed: readiness remains unauthenticated by design and does not change request validation or application state.
Retries, partial failure, and concurrent execution cannot produce unsafe or inconsistent results	Addressed: probes still run independently via Promise.allSettled; a Horizon failure only marks readiness unready and does not suppress other probe results.
Focused tests cover success, rejection, boundary, and regression scenarios	Addressed: tests/healthReady.test.ts adds Horizon success/rejection/timeout/HTTP regression coverage.
Existing callers remain compatible, or the PR includes a tested migration path	Addressed: response shape is additive (checks.horizon) and status semantics remain ready/unready with HTTP 200/503.
Relevant diagnostics make failures diagnosable without exposing sensitive data	Addressed: Horizon probe failures are logged as readiness_horizon_check_failed and returned as health metadata only.

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