feat: Add JS minification to build pipeline and expand test coverage

Task 1: Add JS minification to build pipeline (fix BUG-053)
- Update scripts/build-html.cjs to minify JS files in-place with terser
- Modified public/src/index.html and status.html to reference original JS files
- Add TDD test to verify JS minification works correctly

Task 2: Expand test coverage for untested routes
- Add tests for /v1/usage endpoint (auth required, admin access checks)
- Add tests for /v1/billing/checkout route (rate limiting, config checks)
- Add tests for rate limit headers on PDF conversion endpoints
- Add tests for 404 handler JSON error format for API vs HTML routes
- All tests follow TDD principles (RED → GREEN)

Task 3: Update swagger-jsdoc to fix npm audit vulnerability
- Upgraded swagger-jsdoc to 7.0.0-rc.6
- Resolved minimatch vulnerability via npm audit fix
- Verified OpenAPI generation still works correctly
- All 52 tests passing, 0 vulnerabilities remaining

Build improvements and security hardening complete.

public/src/index.html

@@ -672,6 +672,6 @@ html, body {
   </div>
 </div>
 
-<script src="/app.min.js"></script>
+<script src="/app.js"></script>
 </body>
 </html>

public/src/status.html

@@ -49,6 +49,6 @@
 
 {{> footer}}
 
-<script src="/status.min.js"></script>
+<script src="/status.js"></script>
 </body>
 </html>