refactor(demo): Use handlePdfRoute to reduce boilerplate

- Refactored demo routes to use shared handlePdfRoute utility
- Added handleDemoPdfRoute wrapper to preserve attachment disposition
- Preserved watermark injection and demo.pdf default filename
- Added comprehensive TDD tests for Content-Disposition behavior
- Reduced demo.ts from 269 to 238 lines (31 lines removed)
- All 628 tests pass including 6 new behavioral tests

Fixes duplicated error handling, validation, and concurrency logic
while maintaining existing demo route behavior.

src/__tests__/demo.test.ts

@@ -199,4 +199,74 @@ describe("POST /v1/demo/markdown", () => {
     expect(res.status).toBe(400);
     expect(res.body.error).toMatch(/format/);
   });
+
+  // NEW TDD TESTS - These should verify current behavior before refactoring
+  it("returns Content-Disposition attachment header", async () => {
+    const res = await request(app)
+      .post("/v1/demo/markdown")
+      .set("content-type", "application/json")
+      .send({ markdown: "# Hello" });
+    expect(res.status).toBe(200);
+    expect(res.headers["content-disposition"]).toMatch(/attachment/);
+    expect(res.headers["content-disposition"]).toMatch(/filename="demo\.pdf"/);
+  });
+
+  it("returns custom filename in attachment header", async () => {
+    const res = await request(app)
+      .post("/v1/demo/markdown")
+      .set("content-type", "application/json")
+      .send({ markdown: "# Hello", filename: "custom.pdf" });
+    expect(res.status).toBe(200);
+    expect(res.headers["content-disposition"]).toMatch(/attachment/);
+    expect(res.headers["content-disposition"]).toMatch(/filename="custom\.pdf"/);
+  });
+
+  it("injects watermark into HTML content", async () => {
+    const { renderPdf } = await import("../services/browser.js");
+    const res = await request(app)
+      .post("/v1/demo/markdown")
+      .set("content-type", "application/json")
+      .send({ markdown: "# Hello" });
+    expect(res.status).toBe(200);
+    
+    const calledHtml = vi.mocked(renderPdf).mock.calls[0][0];
+    expect(calledHtml).toContain("Generated by DocFast — docfast.dev");
+    expect(calledHtml).toContain("Upgrade to Pro for clean PDFs");
+    expect(calledHtml).toContain("position:fixed;top:0;left:0;width:100%;height:100%"); // watermark overlay
+  });
+
+  // NEW TDD TESTS - These should verify current behavior before refactoring
+  it("returns Content-Disposition attachment header", async () => {
+    const res = await request(app)
+      .post("/v1/demo/html")
+      .set("content-type", "application/json")
+      .send({ html: "<h1>Hello</h1>" });
+    expect(res.status).toBe(200);
+    expect(res.headers["content-disposition"]).toMatch(/attachment/);
+    expect(res.headers["content-disposition"]).toMatch(/filename="demo\.pdf"/);
+  });
+
+  it("returns custom filename in attachment header", async () => {
+    const res = await request(app)
+      .post("/v1/demo/html")
+      .set("content-type", "application/json")
+      .send({ html: "<h1>Hello</h1>", filename: "custom.pdf" });
+    expect(res.status).toBe(200);
+    expect(res.headers["content-disposition"]).toMatch(/attachment/);
+    expect(res.headers["content-disposition"]).toMatch(/filename="custom\.pdf"/);
+  });
+
+  it("injects watermark into HTML content", async () => {
+    const { renderPdf } = await import("../services/browser.js");
+    const res = await request(app)
+      .post("/v1/demo/html")
+      .set("content-type", "application/json")
+      .send({ html: "<h1>Hello</h1>" });
+    expect(res.status).toBe(200);
+    
+    const calledHtml = vi.mocked(renderPdf).mock.calls[0][0];
+    expect(calledHtml).toContain("Generated by DocFast — docfast.dev");
+    expect(calledHtml).toContain("Upgrade to Pro for clean PDFs");
+    expect(calledHtml).toContain("position:fixed;top:0;left:0;width:100%;height:100%"); // watermark overlay
+  });
 });