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/migrations/migration_lock.toml

@@ -1,3 +1,3 @@
 # Please do not edit this file manually
 # It should be added in your version-control system (e.g., Git)
-provider = "sqlite"
+provider = "postgresql"