refactor: consolidate and debloat calendar sync implementation

Merge duplicated Google/Outlook engines, background handlers, and API
layers into shared calendarSync modules to cut complexity without changing
user-facing sync behavior.

src/seqta/utils/outlookCalendar/upsertEvent.test.ts

@@ -1,48 +1,31 @@
 import { beforeEach, describe, expect, it, jest } from "@jest/globals";
-import { upsertOutlookCalendarEvent, deleteOutlookCalendarEvent } from "./upsertEvent";
 
-const fetchMock = jest.fn();
-global.fetch = fetchMock as unknown as typeof fetch;
+const mockFetch = jest.fn<typeof fetch>();
+global.fetch = mockFetch as typeof fetch;
 
-describe("upsertOutlookCalendarEvent", () => {
+import {
+  deleteOutlookCalendarEvent,
+  upsertOutlookCalendarEvent,
+} from "@/seqta/utils/calendarSync/remoteEvents";
+
+describe("outlook calendar remote events", () => {
   beforeEach(() => {
-    fetchMock.mockReset();
+    mockFetch.mockReset();
   });
 
-  it("creates a new event when no existing id", async () => {
-    fetchMock.mockResolvedValue({
-      ok: true,
-      status: 200,
-      json: async () => ({ id: "evt-1" }),
-    });
+  it("creates a new event when none exists", async () => {
+    mockFetch.mockResolvedValueOnce(
+      new Response(JSON.stringify({ id: "evt-1" }), { status: 201 }),
+    );
 
     const id = await upsertOutlookCalendarEvent("token", undefined, { subject: "Math" });
     expect(id).toBe("evt-1");
-    expect(fetchMock).toHaveBeenCalledWith(
-      "https://graph.microsoft.com/v1.0/me/events",
-      expect.objectContaining({ method: "POST" }),
-    );
+    expect(mockFetch).toHaveBeenCalledTimes(1);
   });
 
-  it("patches when an existing id is provided", async () => {
-    fetchMock.mockResolvedValue({ ok: true, status: 200, json: async () => ({}) });
+  it("deletes an event", async () => {
+    mockFetch.mockResolvedValueOnce(new Response(null, { status: 204 }));
 
-    const id = await upsertOutlookCalendarEvent("token", "evt-1", { subject: "Math" });
-    expect(id).toBe("evt-1");
-    expect(fetchMock).toHaveBeenCalledWith(
-      "https://graph.microsoft.com/v1.0/me/events/evt-1",
-      expect.objectContaining({ method: "PATCH" }),
-    );
-  });
-});
-
-describe("deleteOutlookCalendarEvent", () => {
-  beforeEach(() => {
-    fetchMock.mockReset();
-  });
-
-  it("treats 404 as success", async () => {
-    fetchMock.mockResolvedValue({ ok: false, status: 404, json: async () => ({}) });
     await expect(deleteOutlookCalendarEvent("token", "evt-1")).resolves.toBeUndefined();
   });
 });