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>

.gitignore

@@ -42,9 +42,8 @@ yarn-error.log*
 *.tsbuildinfo
 next-env.d.ts
 
-# prisma — generated client is committed so Turbopack can resolve it
-# during next build (Turbopack excludes gitignored paths from module resolution).
-# Run `npx prisma generate` after schema changes to update.
+# prisma — generated client lives in node_modules after prisma generate
+src/generated
 
 # ide
 .idea