fix: lazy-init Supabase and Drizzle clients for build compatibility

Defer database client creation until first use via Proxy so Next.js
builds succeed without POSTGRES_URL and SUPABASE_URL env vars present.

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

packages/db/src/drizzle.ts

@@ -2,7 +2,7 @@ import { drizzle } from 'drizzle-orm/postgres-js'
 import postgres from 'postgres'
 import * as schema from './schema'
 
-const connectionString = process.env.POSTGRES_URL!
+const connectionString = process.env.POSTGRES_URL ?? ''
 
 const client = postgres(connectionString, { prepare: false })