fix(backend): load DATABASE_URL for Prisma CLI from repo root .env

Use dotenv-cli for db:push, db:seed, db:studio, and migrate:deploy so they
match the API (root .env). Load dotenv in seed.ts for direct runs.
Document absolute DATABASE_URL in .env.example.

Made-with: Cursor

backend/prisma/seed.ts

@@ -1,5 +1,9 @@
+import dotenv from 'dotenv';
+import path from 'path';
 import { PrismaClient } from '@prisma/client';
 
+dotenv.config({ path: path.resolve(__dirname, '../../.env') });
+
 const prisma = new PrismaClient();
 
 async function main() {