Switch database from SQLite to PostgreSQL

Replace in-memory JSON file storage with Prisma ORM using PostgreSQL.
Update environment variables and Prisma schema accordingly.

prisma/schema.prisma

@@ -4,7 +4,7 @@ generator client {
 }
 
 datasource db {
-  provider = "sqlite"
+  provider = "postgresql"
   url      = env("DATABASE_URL")
 }