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_head_incremental.py

@@ -17,28 +17,12 @@ from backend.app.services.ml.heads import (
     _head_fingerprints,
     _heads_needing_retrain,
 )
+from tests.factories import make_image as _img
+from tests.factories import make_tag as _tag
 
 pytestmark = pytest.mark.integration
 
 
-def _img(db, sha: str) -> 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",
-    )
-    db.add(img)
-    db.flush()
-    return img
-
-
-def _tag(db, name: str) -> Tag:
-    t = Tag(name=name, kind=TagKind.general)
-    db.add(t)
-    db.flush()
-    return t
-
-
 def _apply(db, image_id: int, tag_id: int) -> None:
     db.execute(image_tag.insert().values(
         image_record_id=image_id, tag_id=tag_id, source="manual",