test(core): replace image-cache module mocks with globalThis.fetch spies

Switch `mock.module("../src/image-cache", ...)` stubs in thumbhash, person-detail, colors, and credits tests to `spyOn(globalThis, "fetch")` instead. This avoids the cross-file persistence problem with `mock.module` and tests the real code paths more faithfully.

Update `image-cache.ts` to call `globalThis.fetch` so the spy intercepts it, add `mock.restore()` to the global `afterEach` in `preload.ts`, and set `IMAGE_CACHE_ENABLED=false` in affected tests to avoid disk writes during test runs.

packages/core/test/credits.test.ts

@@ -1,12 +1,7 @@
-import { beforeEach, describe, expect, mock, test } from "bun:test";
+import { beforeEach, describe, expect, test } from "bun:test";
 import { persons, titleCast } from "@sofa/db/schema";
 import { clearAllTables, insertTitle, testDb } from "@sofa/db/test-utils";
 
-mock.module("../src/image-cache", () => ({
-  imageCacheEnabled: () => false,
-  cacheProfilePhotos: async () => {},
-}));
-
 import { getCastForTitle } from "../src/credits";
 
 beforeEach(() => {