Add Prisma 7 PostgreSQL adapter

Prisma 7 requires explicit database adapter for runtime connections.
- Install @prisma/adapter-pg + pg
- Update prisma.ts: use PrismaPg adapter with DATABASE_URL
- Update seed.ts: same adapter pattern
- Dockerfile: copy pg deps to runner stage

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

prisma/seed.ts

@@ -1,8 +1,10 @@
 import "dotenv/config";
 import { PrismaClient } from "../src/generated/prisma/client";
+import { PrismaPg } from "@prisma/adapter-pg";
 import bcrypt from "bcryptjs";
 
-const prisma = new PrismaClient();
+const adapter = new PrismaPg({ connectionString: process.env.DATABASE_URL! });
+const prisma = new PrismaClient({ adapter });
 
 async function main() {
   console.log("Seeding database...");