feat(ui): confirm/keep auto-applied tags (milestone 139)

Auto-applied tags are provisional (they don't train the model + can be retracted
until confirmed), so surface and confirm them:
- Backend: list_for_image + get_image_with_tags now include `source` + a
  `confirmed` flag on each applied tag (via serialize_tag, image-scoped; defaulted
  for autocomplete/directory callers).
- Frontend: TagChip badges an unconfirmed auto-tag with an "auto" pill + a
  one-click Keep/confirm (✓) → POST /images/<id>/tags/<id>/confirm, which promotes
  it to a training positive and shields it from the retraction sweep; TagPanel
  reloads so the badge + button drop once confirmed.
Contract test for the source/confirmed payload.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

tests/test_api_tags.py

@@ -141,6 +141,38 @@ async def test_applied_tag_grounding_returns_winning_region(client, db):
     assert body["grounding"]["kind"] == "concept"
 
 
+@pytest.mark.asyncio
+async def test_applied_tags_expose_source_and_confirmed(client, db):
+    # The chip UI needs each applied tag's source (auto vs manual) + confirmed
+    # state to badge auto-tags and offer Keep/confirm (milestone 139).
+    from backend.app.models import TagPositiveConfirmation
+    from backend.app.models.tag import image_tag
+
+    img = ImageRecord(
+        path="/images/srcflag.jpg", sha256="sf" * 32, size_bytes=1,
+        mime="image/jpeg", width=1, height=1, origin="imported_filesystem",
+        integrity_status="unknown",
+    )
+    db.add(img)
+    await db.flush()
+    auto = await TagService(db).find_or_create("autotag", TagKind.general)
+    manual = await TagService(db).find_or_create("manualtag", TagKind.general)
+    await db.execute(image_tag.insert().values(
+        image_record_id=img.id, tag_id=auto.id, source="head_auto"))
+    await db.execute(image_tag.insert().values(
+        image_record_id=img.id, tag_id=manual.id, source="manual"))
+    db.add(TagPositiveConfirmation(image_record_id=img.id, tag_id=manual.id))
+    await db.commit()
+
+    resp = await client.get(f"/api/images/{img.id}/tags")
+    assert resp.status_code == 200
+    by_id = {t["id"]: t for t in await resp.get_json()}
+    assert by_id[auto.id]["source"] == "head_auto"
+    assert by_id[auto.id]["confirmed"] is False
+    assert by_id[manual.id]["source"] == "manual"
+    assert by_id[manual.id]["confirmed"] is True     # has a confirmation row
+
+
 @pytest.mark.asyncio
 async def test_applied_tag_grounding_no_head(client, db):
     # A tag with no head can't be localized → has_head False, grounding null; the