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>

package.json

@@ -3,6 +3,7 @@
   "version": "0.1.0",
   "private": true,
   "scripts": {
+    "postinstall": "prisma generate",
     "dev": "next dev",
     "build": "next build",
     "start": "next start",