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>

src/lib/prisma.ts

@@ -1,10 +1,17 @@
 import { PrismaClient } from "@/generated/prisma/client";
+import { PrismaPg } from "@prisma/adapter-pg";
 
 const globalForPrisma = globalThis as unknown as {
   prisma: PrismaClient | undefined;
 };
 
-// In Prisma 7, connection is configured via prisma.config.ts, not in constructor
-export const prisma = globalForPrisma.prisma ?? new PrismaClient();
+function createPrismaClient() {
+  const adapter = new PrismaPg({
+    connectionString: process.env.DATABASE_URL!,
+  });
+  return new PrismaClient({ adapter });
+}
+
+export const prisma = globalForPrisma.prisma ?? createPrismaClient();
 
 if (process.env.NODE_ENV !== "production") globalForPrisma.prisma = prisma;