chore(db): migrate from SQLite to PostgreSQL

- Change Prisma provider from sqlite to postgresql
- Replace better-sqlite3 adapter with @prisma/adapter-pg
- Update prisma.config.ts and lib/prisma.ts for PostgreSQL
- Add DATABASE_URL to .env.local
- Regenerate Prisma client for PostgreSQL
- Recreate migrations for PostgreSQL database

prisma/schema.prisma

@@ -4,7 +4,7 @@ generator client {
 }
 
 datasource db {
-  provider = "sqlite"
+  provider = "postgresql"
 }
 
 model User {