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

tests/test_process_auto_apply.py

@@ -16,6 +16,7 @@ from backend.app.models import (
 from backend.app.models.tag import image_tag
 from backend.app.services.ml.heads import system_tag_auto_apply_sweep
 from backend.app.services.ml.training_data import _ids_with_tag
+from tests.factories import make_image as _img
 
 pytestmark = pytest.mark.integration
 
@@ -26,17 +27,6 @@ def _emb(slot: int) -> list[float]:
     return v
 
 
-def _img(db, sha: str, emb) -> ImageRecord:
-    img = ImageRecord(
-        path=f"/images/{sha}.jpg", sha256=sha, size_bytes=1, mime="image/jpeg",
-        width=1, height=1, origin="imported_filesystem",
-        integrity_status="unknown", siglip_embedding=emb,
-    )
-    db.add(img)
-    db.flush()
-    return img
-
-
 def _head(db, tag_id: int, slot: int, *, weight=1.0):
     s = db.execute(select(MLSettings).where(MLSettings.id == 1)).scalar_one()
     w = [0.0] * 1152