feat: enhance health check endpoint to verify all upstream dependencies

[charannyk06]

Priority: P3 — Low

Problem

chainreview-api/src/routes/health.ts exists but likely returns a minimal response (e.g., { status: 'ok' }). A meaningful health check should verify connectivity to all upstream dependencies.

Solution

Enhance the health endpoint to check:

GET /health

Response:
{
  "status": "healthy" | "degraded" | "unhealthy",
  "version": "1.2.3",
  "checks": {
    "database": { "status": "healthy", "latency_ms": 12 },
    "supabase": { "status": "healthy", "latency_ms": 45 },
    "anthropic": { "status": "healthy", "latency_ms": 120 },
    "stripe": { "status": "healthy", "latency_ms": 89 }
  },
  "timestamp": "2026-03-05T10:00:00Z"
}

Implementation

• Database: Run a simple SELECT 1 query
• Supabase: Check auth service liveness
• Anthropic: Optional lightweight check (or just verify key format to avoid unnecessary API calls)
• Stripe: Optional check against Stripe's status API

Behavior

• Overall status = "degraded" if any non-critical check fails
• Overall status = "unhealthy" if critical checks (database) fail
• Respond with 200 for "healthy"/"degraded", 503 for "unhealthy"
• Cache responses for 10 seconds to prevent hammering upstream services

Use cases

• Cloudflare Workers health checks
• Uptime monitoring (UptimeRobot, Better Stack)
• Kubernetes liveness/readiness probes (if self-hosted)

Acceptance Criteria

• /health returns individual dependency check results
• Returns 503 when critical dependencies are down
• Response cached for 10 seconds
• Version included in response