fix: use webpack build in CI to bypass Turbopack resolution bug

Turbopack cannot resolve imports from src/generated/prisma even
with relative paths and committed files. Switch to --webpack for
CI build. Also add file verification step for debugging.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -47,8 +47,17 @@ jobs:
       - name: Run database migrations
         run: npx prisma migrate deploy
 
+      - name: Verify generated files exist
+        run: |
+          echo "=== src/generated/prisma contents ==="
+          ls -la src/generated/prisma/
+          echo "=== index.ts content ==="
+          cat src/generated/prisma/index.ts
+          echo "=== client.ts first 3 lines ==="
+          head -3 src/generated/prisma/client.ts
+
       - name: Build (includes type check and lint)
-        run: npm run build
+        run: npx next build --webpack
         env:
           AUTH_SECRET: ci-test-secret-not-for-production
           AUTH_TRUST_HOST: "true"