feat: add PDF render timing to convert and demo routes

- renderPdf() and renderUrlPdf() now return { pdf, durationMs }
- Timing wraps the actual render with Date.now()
- Log render duration via logger.info
- Add X-Render-Time response header in convert and demo routes
- Update all callers in convert, demo, templates routes
- Add TDD tests in render-timing.test.ts
- Update existing test mocks for new return shape

src/__tests__/templates-render-validation.test.ts

@@ -16,7 +16,7 @@ beforeEach(async () => {
   vi.clearAllMocks();
 
   const { renderPdf } = await import("../services/browser.js");
-  vi.mocked(renderPdf).mockResolvedValue(Buffer.from("%PDF-1.4 mock"));
+  vi.mocked(renderPdf).mockResolvedValue({ pdf: Buffer.from("%PDF-1.4 mock"), durationMs: 10 });
 
   const { authMiddleware } = await import("../middleware/auth.js");
   const { usageMiddleware } = await import("../middleware/usage.js");