fix: use standard @prisma/client imports instead of custom output path

The custom generator output to src/generated/prisma caused persistent
Turbopack module resolution failures in CI. Switch to the standard
@prisma/client import path which all bundlers resolve correctly.

- Remove custom output from prisma schema generator
- Update all imports from ../generated/prisma to @prisma/client
- Add postinstall script to auto-run prisma generate after npm ci
- Remove generated files from git (no longer needed in source tree)
- Simplify CI workflow (remove verify step and --webpack workaround)

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

prisma/schema.prisma

@@ -1,6 +1,5 @@
 generator client {
   provider = "prisma-client"
-  output   = "../src/generated/prisma"
 }
 
 datasource db {

prisma/seed.ts

@@ -1,4 +1,4 @@
-import { PrismaClient } from "../src/generated/prisma";
+import { PrismaClient } from "@prisma/client";
 import { PrismaPg } from "@prisma/adapter-pg";
 import pg from "pg";
 import { hash } from "bcryptjs";