docs(health): document all health endpoint response fields and states

[Ajibose]

Closes #571

Summary

• Expands docs/health-endpoints.md with complete field-by-field documentation for all four health endpoints: liveness (/health), readiness (/health/ready), diagnostics (/health/detailed), and indexer heartbeat (/health/indexer)
• Defines healthy / degraded / unhealthy states and allowed values for every field
• Explains the liveness vs readiness distinction with a comparison table
• Documents the exact non-200 trigger condition for each endpoint
• Adds health.response-schema.test.ts — 70 tests that act as a living contract between the code and the documentation
• Fixes a pre-existing setHeader is not a function bug in two existing test files that caused 6 tests to fail on main

New files

• src/modules/health/health.response-schema.test.ts — response field schema tests for all four health endpoints

Modified files

• docs/health-endpoints.md — expanded from partial docs to complete endpoint reference
• src/modules/health/health.controllers.test.ts — added res.setHeader stub to mockResponse helper (fixes 6 pre-existing failures)
• src/modules/health/health.controllers.integration.test.ts — same setHeader fix

Implementation details

Documentation (docs/health-endpoints.md)

The updated doc covers:

Endpoint	Non-200 trigger
GET /health (liveness)	Never — always 200
GET /health/ready (readiness)	ready: false when any check is "fail" → 503
GET /health/detailed (diagnostics)	database.status === "disconnected" in production → 503
GET /health/indexer (worker heartbeat)	data.status === "degraded" → 503

Each endpoint section includes: response shape example, field table with types and descriptions, state tables mapping field values to conditions, and the precise non-200 trigger condition.

The liveness vs readiness distinction is explained in a comparison table at the top of the document.

Tests (health.response-schema.test.ts)

Tests are organised by endpoint and state. Key coverage:

• Liveness — response shape, field types, always-200 contract
• Readiness (healthy) — all checks pass, field types, per-check latencyMs
• Readiness (degraded) — 503 on any failure, ready: false as trigger, error string present, latencyMs still present at top level
• Detailed (DB connected) — all top-level fields, memory, system, timeouts, database, syncing computation, services array values
• Detailed (DB disconnected, non-production) — 200 still returned, database.status: "disconnected", no responseTime, Database service "unhealthy"
• Indexer heartbeat — all three status states (unknown, healthy, degraded) with HTTP code, lastSuccessfulRun, and staleSinceMs assertions
• Heartbeat record — response shape on POST /health/indexer/heartbeat

How to test

# Run only health module tests
pnpm exec jest src/modules/health/ --no-coverage

# Expected: 91 tests passed across 4 suites