test(server): add startServer PAPERCLIP_API_URL guard tests

Add integration tests that directly exercise the PAPERCLIP_API_URL
conditional guard in startServer():
- Externally set PAPERCLIP_API_URL is preserved (not overwritten)
- Unset PAPERCLIP_API_URL falls back to host-based URL

Addresses Greptile review comment on PR #3472.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

server/src/__tests__/server-startup-feedback-export.test.ts

@@ -171,3 +171,27 @@ describe("startServer feedback export wiring", () => {
     });
   });
 });
+
+describe("startServer PAPERCLIP_API_URL handling", () => {
+  beforeEach(() => {
+    vi.clearAllMocks();
+    process.env.BETTER_AUTH_SECRET = "test-secret";
+    delete process.env.PAPERCLIP_API_URL;
+  });
+
+  it("uses the externally set PAPERCLIP_API_URL when provided", async () => {
+    process.env.PAPERCLIP_API_URL = "http://custom-api:3100";
+
+    const started = await startServer();
+
+    expect(started.apiUrl).toBe("http://custom-api:3100");
+    expect(process.env.PAPERCLIP_API_URL).toBe("http://custom-api:3100");
+  });
+
+  it("falls back to host-based URL when PAPERCLIP_API_URL is not set", async () => {
+    const started = await startServer();
+
+    expect(started.apiUrl).toBe("http://127.0.0.1:3210");
+    expect(process.env.PAPERCLIP_API_URL).toBe("http://127.0.0.1:3210");
+  });
+});