fix: use bun:sqlite in production, better-sqlite3 as test-only devDep

Production (Docker): src/lib/db.ts now uses bun:sqlite + drizzle-orm/bun-sqlite.
No native addon compilation needed — bun:sqlite is a Bun built-in. The Dockerfile
drops all native build tools (python3, make, g++) and uses --ignore-scripts.

Tests (Vitest/Node.js): bun:sqlite is unavailable under Node.js, so:
- tests/helpers/db.ts keeps better-sqlite3 + drizzle-orm/better-sqlite3 for
  integration tests that need a real in-memory SQLite
- vitest.config.ts aliases bun:sqlite → a thin better-sqlite3 shim and
  drizzle-orm/bun-sqlite → drizzle-orm/better-sqlite3 for unit tests that
  transitively import src/lib/db.ts without executing any queries
- better-sqlite3 stays as a devDependency (test-only, not built in Docker)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

next.config.mjs

@@ -2,6 +2,7 @@
 
 /** @type {import('next').NextConfig} */
 const nextConfig = {
+  serverExternalPackages: ['bun:sqlite'],
   experimental: {
     serverActions: {
       bodySizeLimit: '2mb'