Add prisma generate as postinstall script

Created by: jedmund

Summary

• Adds "postinstall": "prisma generate" to package.json scripts
• Ensures Prisma client types are generated automatically after pnpm install, preventing ~40 missing type export errors from @prisma/client

Test plan

• Run pnpm install and verify prisma generate runs automatically
• Verify pnpm run check no longer reports missing Prisma type exports