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>

package.json

@@ -13,11 +13,13 @@
     "seed": "ts-node --project tsconfig.json prisma/seed.ts"
   },
   "dependencies": {
+    "@prisma/adapter-pg": "^7.6.0",
     "@prisma/client": "^7.6.0",
     "bcryptjs": "^3.0.3",
     "better-auth": "^1.6.0",
     "clsx": "^2.1.1",
     "next": "16.2.2",
+    "pg": "^8.20.0",
     "react": "19.2.4",
     "react-dom": "19.2.4",
     "resend": "^6.10.0",
@@ -28,6 +30,7 @@
     "@tailwindcss/postcss": "^4",
     "@types/bcryptjs": "^2.4.6",
     "@types/node": "^20",
+    "@types/pg": "^8.20.0",
     "@types/react": "^19",
     "@types/react-dom": "^19",
     "dotenv": "^17.4.1",