perf: investigate DB latency exceeding 500ms threshold for 2 consecutive weeks

[sw-factory-automations]

Description

The health endpoint DB latency has exceeded the 500ms threshold for two consecutive weeks: 799ms in W19, 658ms in W20. The health check already uses best-of-2 sampling with direct fetch (bypassing the Supabase JS client), so the bottleneck is likely on the Supabase infrastructure side — regional distance, connection pool saturation, or cold starts on the PostgREST layer.

Source: metrics/weekly/2026-W20-perf.md

Acceptance Criteria

• Profile the health endpoint latency breakdown: DNS resolution, TCP/TLS handshake, server processing, response transfer
• Check Supabase dashboard for connection pool utilization and query performance metrics
• Determine if the Vercel deployment region and Supabase project region are co-located (same AWS region)
• If regions differ, document the expected latency floor and whether migration is feasible
• If regions match, investigate PostgREST cold start behavior and whether connection pooling (PgBouncer/Supavisor) is enabled
• Achieve DB latency ≤ 500ms on the health endpoint, or document why the threshold should be adjusted
• pnpm lint && pnpm typecheck && pnpm test pass

Dependencies

None

Technical Notes

• Health endpoint: src/app/api/health/route.ts — already optimized with direct fetch, best-of-2 sampling, keep-alive headers
• The health_check RPC function is a simple DB ping — the latency is dominated by network round-trip, not query execution
• Supabase project config and Vercel deployment region are infrastructure settings, not code changes
• If the fix requires Supabase region migration or plan changes, document the steps for a maintainer to execute

Approval Required

This is a HIGH risk investigation:

• May require infrastructure changes (Supabase region migration, connection pooling configuration)
• Could involve Supabase plan changes with cost implications
• Region migration would require downtime planning and data migration

Comment "approved" to release this to the automation queue.