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/demo.ts

@@ -133,7 +133,7 @@ router.post("/html", async (req: Request & { acquirePdfSlot?: () => Promise<void
       printBackground: true,
       margin: { top: "0", right: "0", bottom: "0", left: "0" },
     };
-    const pdf = await renderPdf(fullHtml, {
+    const { pdf, durationMs } = await renderPdf(fullHtml, {
       ...defaultOpts,
       ...validation.sanitized,
     });
@@ -142,6 +142,7 @@ router.post("/html", async (req: Request & { acquirePdfSlot?: () => Promise<void
     res.setHeader("Content-Type", "application/pdf");
     res.setHeader("Content-Disposition", `attachment; filename="${filename}"`);
     res.setHeader("Content-Length", pdf.length);
+    res.setHeader("X-Render-Time", String(durationMs));
     res.send(pdf);
   } catch (err: any) {
     if (err.message === "QUEUE_FULL") {
@@ -237,7 +238,7 @@ router.post("/markdown", async (req: Request & { acquirePdfSlot?: () => Promise<
       printBackground: true,
       margin: { top: "0", right: "0", bottom: "0", left: "0" },
     };
-    const pdf = await renderPdf(fullHtml, {
+    const { pdf, durationMs } = await renderPdf(fullHtml, {
       ...defaultOpts,
       ...validation.sanitized,
     });
@@ -246,6 +247,7 @@ router.post("/markdown", async (req: Request & { acquirePdfSlot?: () => Promise<
     res.setHeader("Content-Type", "application/pdf");
     res.setHeader("Content-Disposition", `attachment; filename="${filename}"`);
     res.setHeader("Content-Length", pdf.length);
+    res.setHeader("X-Render-Time", String(durationMs));
     res.send(pdf);
   } catch (err: any) {
     if (err.message === "QUEUE_FULL") {