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/routes/templates.ts

@@ -166,7 +166,7 @@ templatesRouter.post("/:id/render", async (req: Request, res: Response) => {
     const sanitizedPdf = { format: "A4" as string, ...validation.sanitized };
 
     const html = renderTemplate(id, data);
-    const pdf = await renderPdf(html, sanitizedPdf);
+    const { pdf, durationMs } = await renderPdf(html, sanitizedPdf);
 
     const filename = sanitizeFilename(data._filename || `${id}.pdf`);
     res.setHeader("Content-Type", "application/pdf");