fix: prisma url in schema + first user becomes ADMIN

- Add url = env("DATABASE_URL") back to schema (required by Prisma 7.5)
- Simplify prisma.config.ts
- First registered user auto-promoted to ADMIN via databaseHooks
- Subsequent users get HOST role

prisma/schema.prisma

@@ -9,6 +9,7 @@ generator client {
 
 datasource db {
   provider = "postgresql"
+  url      = env("DATABASE_URL")
 }
 
 // ======================== ENUMS =============================