Bound dashboard database pools for Vercel

[suguanYang]

Summary

• bound the main dashboard PostgreSQL pool to 2 connections per app instance
• bound the independent newsletter pool to 1 connection per app instance
• configure 10-second idle and 5-second acquisition timeouts through validated environment variables
• register both pools with the Vercel Functions lifecycle when running on Vercel
• document the pool settings and cover defaults, validation, lifecycle registration, and both database consumers

This is the application-code prerequisite for Ontos-AI/knowhere-api-infra#20. It does not create or modify Vercel projects, environment values, domains, DNS, or other cloud resources. Existing staging GitHub and Google OAuth clients remain reusable because staging.knowhereto.ai and its callback URLs do not change.

Validation

• pnpm test — 27 files, 114 tests passed
• pnpm type-check — passed
• focused Biome check for all changed source/config files — passed
• pnpm build with CI-style placeholder public environment values — passed
• two-axis review against repository standards and issue UI enhancement #20 — no remaining findings

Known repository baseline

The repository-wide pnpm lint remains blocked by an unrelated existing unused product variable in app/(landing)/versus/[product]/page.tsx, plus existing warnings. The changed files pass Biome.

Closes no issue; issue #20 still requires separately approved Vercel configuration, deployment, and live staging acceptance.