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__/convert.test.ts

@@ -15,8 +15,8 @@ beforeEach(async () => {
   vi.resetModules();
 
   const { renderPdf, renderUrlPdf } = await import("../services/browser.js");
-  vi.mocked(renderPdf).mockResolvedValue(Buffer.from("%PDF-1.4 mock"));
-  vi.mocked(renderUrlPdf).mockResolvedValue(Buffer.from("%PDF-1.4 mock url"));
+  vi.mocked(renderPdf).mockResolvedValue({ pdf: Buffer.from("%PDF-1.4 mock"), durationMs: 10 });
+  vi.mocked(renderUrlPdf).mockResolvedValue({ pdf: Buffer.from("%PDF-1.4 mock url"), durationMs: 10 });
 
   const dns = await import("node:dns/promises");
   vi.mocked(dns.default.lookup).mockResolvedValue({ address: "93.184.216.34", family: 4 } as any);