fix: revert swagger-jsdoc to 6.2.8 (7.0.0-rc.6 broke OpenAPI spec generation) + add OpenAPI spec tests

swagger-jsdoc 7.0.0-rc.6 returns empty spec (0 paths), breaking /docs and /openapi.json.
Reverted to 6.2.8 which correctly generates all 10+ paths.
Added 2 regression tests to catch this in CI.

src/__tests__/api.test.ts

@@ -585,6 +585,27 @@ describe("Rate limit headers on PDF endpoints", () => {
   });
 });
 
+describe("OpenAPI spec", () => {
+  it("returns a valid OpenAPI 3.0 spec with paths", async () => {
+    const res = await fetch(`${BASE}/openapi.json`);
+    expect(res.status).toBe(200);
+    const spec = await res.json();
+    expect(spec.openapi).toBe("3.0.3");
+    expect(spec.info).toBeDefined();
+    expect(spec.info.title).toBe("DocFast API");
+    expect(Object.keys(spec.paths).length).toBeGreaterThanOrEqual(8);
+  });
+
+  it("includes all major endpoint groups", async () => {
+    const res = await fetch(`${BASE}/openapi.json`);
+    const spec = await res.json();
+    const paths = Object.keys(spec.paths);
+    expect(paths).toContain("/v1/convert/html");
+    expect(paths).toContain("/v1/convert/markdown");
+    expect(paths).toContain("/health");
+  });
+});
+
 describe("404 handler", () => {
   it("returns proper JSON error format for API routes", async () => {
     const res = await fetch(`${BASE}/v1/nonexistent-endpoint`);