Move Prisma config to project root as .mjs for Prisma 7

- Move prisma.config.ts to prisma.config.mjs at project root
- Use dynamic dotenv import (works in Docker without dotenv)
- Add dotenv to devDependencies for local development
- Install dotenv globally in Docker for migration support

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

prisma/prisma.config.ts

@@ -1,12 +0,0 @@
-import 'dotenv/config'
-import { defineConfig, env } from 'prisma/config'
-
-export default defineConfig({
-  schema: 'prisma/schema.prisma',
-  migrations: {
-    path: 'prisma/migrations',
-  },
-  datasource: {
-    url: env('DATABASE_URL'),
-  },
-})