docs: refresh database setup and env reference

[JasperZhou11]

Summary

• correct the contributor guide from SQLite to the shipped Postgres/pglite tiers
• document pglite boot-time migration behavior and the external-Postgres migration command
• add the pool-mode override and DB watchdog controls to .env.example
• remove a stale better-sqlite3-specific Docker ignore comment while preserving the exclusion

Verification

• corepack pnpm -r typecheck
• corepack pnpm --filter @loopany/server test src/server/boot.test.ts
• corepack pnpm --filter @loopany/server test src/env.test.ts src/server/dbWatchdog.test.ts src/db/poolOptions.test.ts
• git diff --check
• relative Markdown/image link validation