refactor: test row factories and the fetch stub have one copy each (3109)

tests/factories.py holds image_row/make_image/make_image_async/make_tag. The
17 byte-identical _img/_tag helpers (15 modules) now import them under their
old names, so no call site changed. frontend/test/support/stubFetch.js
replaces 15 copies that differed only in formatting. Copies whose bodies
differ (other defaults, other columns, a url-only stub) are left as they
are; folding those needs a look at each caller.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

frontend/test/sources.spec.js

@@ -1,17 +1,7 @@
 import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest'
 import { setActivePinia, createPinia } from 'pinia'
 import { useSourcesStore } from '../src/stores/sources.js'
-
-function stubFetch(handler) {
-  globalThis.fetch = vi.fn(async (url, init) => {
-    const { status, body } = handler(url, init)
-    return {
-      ok: status >= 200 && status < 300,
-      status, statusText: String(status),
-      text: async () => (body == null ? '' : JSON.stringify(body)),
-    }
-  })
-}
+import { stubFetch } from './support/stubFetch.js'
 
 describe('sources store', () => {
   beforeEach(() => setActivePinia(createPinia()))