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.config.ts

@@ -4,9 +4,4 @@ import { defineConfig } from "prisma/config";
 export default defineConfig({
   earlyAccess: true,
   schema: path.join(__dirname, "prisma", "schema.prisma"),
-  migrate: {
-    async url() {
-      return process.env.DATABASE_URL!;
-    },
-  },
-} as any);
+});