feat(fc2b): add tag rename (PATCH /api/tags/<id>) + /api/ml admin

Rename returns 409 on collision (frontend shows FC-2c merge hint), 400
on empty. ml_admin: GET/PATCH settings (thresholds + min_reference;
model versions read-only here), POST backfill / recompute-centroids
returning 202 + celery task id. Tests integration-marked, eager Celery.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_api_ml_admin.py

@@ -0,0 +1,67 @@
+import pytest
+
+from backend.app import create_app
+from backend.app.celery_app import celery
+from backend.app.models import TagKind
+from backend.app.services.tag_service import TagService
+
+pytestmark = pytest.mark.integration
+
+
+@pytest.fixture(autouse=True)
+def eager():
+    celery.conf.task_always_eager = True
+    yield
+    celery.conf.task_always_eager = False
+
+
+@pytest.fixture
+async def app():
+    return create_app()
+
+
+@pytest.fixture
+async def client(app):
+    async with app.test_client() as c:
+        yield c
+
+
+@pytest.mark.asyncio
+async def test_get_and_patch_settings(client):
+    resp = await client.get("/api/ml/settings")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["suggestion_threshold_general"] == pytest.approx(0.95)
+
+    resp = await client.patch(
+        "/api/ml/settings", json={"suggestion_threshold_general": 0.90}
+    )
+    assert resp.status_code == 200
+    assert (await resp.get_json())["suggestion_threshold_general"] == pytest.approx(0.90)
+
+
+@pytest.mark.asyncio
+async def test_backfill_and_recompute_trigger(client):
+    r1 = await client.post("/api/ml/backfill")
+    assert r1.status_code == 202
+    r2 = await client.post("/api/ml/recompute-centroids")
+    assert r2.status_code == 202
+
+
+@pytest.mark.asyncio
+async def test_rename_tag(client, db):
+    tag = await TagService(db).find_or_create("oldname", TagKind.character)
+    await db.commit()
+    resp = await client.patch(f"/api/tags/{tag.id}", json={"name": "New Name"})
+    assert resp.status_code == 200
+    assert (await resp.get_json())["name"] == "New Name"
+
+
+@pytest.mark.asyncio
+async def test_rename_collision_409(client, db):
+    svc = TagService(db)
+    await svc.find_or_create("Taken", TagKind.character)
+    other = await svc.find_or_create("Mover", TagKind.character)
+    await db.commit()
+    resp = await client.patch(f"/api/tags/{other.id}", json={"name": "Taken"})
+    assert resp.status_code == 409