feat(trainer): phase-B body-type classifier — trainer job on the collector host + the classifier stage on the booth

apps/trainer (parking-trainer): inspect / train / evaluate / publish. Reads the wash
collector's SQLite + crops read-only off its volume; time split (validation = newest
slice); thin classes dropped; damped class weights; `features` mode (frozen ImageNet
backbone, on-disk feature cache, seconds to retrain) and `finetune` mode (light
augmentation). CPU-only torch from PyTorch's wheel index. ONNX export checked against
the torch model; NO model file below the validation floor (exit 3, report still written);
exit 2 = not enough labels. `evaluate` scores a shipped model on labels reviewed after
training + the unlabelled pile; `publish` PUTs a version folder to a Gitea generic package.
Light core deps; the `train` extra is heavy — CI syncs without it, torch tests skip.

apps/vision: BodyTypeClassifier (bodytype.onnx + sidecar = the preprocessing contract:
crop margin, input size, RGB 0-255, normalisation inside the graph) and
RefinedVehicleDetector over YOLOX — refines only `car` or a class the classifier trained
on, min-confidence, `detector_class` on the result; path set but no file = phase B off
without an error; a broken file is a health detail. models/bodytype.version (tracked,
empty) pins the published version the Dockerfile fetches at build (BuildKit secret;
a pin that cannot be fetched fails the build). Verified: a trainer model gives identical
probabilities inside the vision service; both images built and smoke-tested.

Delivery: parking-trainer image in build-images.yml, the `trainer` compose profile on the
collector stack (CPU, read-only data, TRAINER_OUT), commented TRAINER_OUT/PUBLISH_TOKEN in
the wash-collector stack, .dockerignore for both Python contexts, trainer deps synced in CI.

Wiki: bodytype-classifier-training rewritten as built (+ one fleet model not per site,
secrets/access, where the crops live), opencv-anpr-service §Phase B, vision-review-outbox,
vision-service-packaging, fleet-deployment-komodo, index, log.

Claude-Session: https://claude.ai/code/session_01FWncR69HgGPuei1dLrW3cU

apps/vision/tests/test_vehicle.py

@@ -143,3 +143,135 @@ def test_app_reports_a_missing_model_file_and_keeps_serving() -> None:
             assert res.json()["vehicle"] is None
     finally:
         os.environ.pop("VISION_VEHICLE_MODEL_PATH", None)
+
+
+# ----------------------------------------------------------------------------------
+# Phase B: the classifier stage over the detector
+# ----------------------------------------------------------------------------------
+
+
+def test_crop_vehicle_adds_the_margin_clamps_and_blurs_the_plate() -> None:
+    cv2 = pytest.importorskip("cv2")
+    from vision_service.vehicle import crop_vehicle
+
+    frame = np.zeros((100, 200, 3), dtype=np.uint8)
+    frame[40:50, 90:110] = (0, 255, 0)  # a green "plate"
+    box = BBox(x1=50, y1=20, x2=150, y2=80)  # 100×60 → 8 % margin = 8 / 5 px
+    crop = crop_vehicle(frame, box, None, 0.08)
+    assert crop.shape == (70, 116, 3)
+    edge = crop_vehicle(frame, BBox(x1=0, y1=0, x2=100, y2=60), None, 0.08)
+    assert edge.shape == (65, 108, 3)  # clamped at the frame's top-left
+    assert crop_vehicle(frame, BBox(x1=10, y1=10, x2=12, y2=12), None, 0.08) is None
+    blurred = crop_vehicle(frame, box, BBox(x1=90, y1=40, x2=110, y2=50), 0.08)
+    strip = blurred[40 - 20 + 5 : 50 - 20 + 5, 90 - 50 + 8 : 110 - 50 + 8, 1]  # plate strip, green channel
+    assert strip.mean() < 200 and crop[20 + 5 : 30 + 5, 40 + 8 : 60 + 8, 1].mean() == 255
+    assert cv2 is not None
+
+
+class FakeClassifier:
+    ready = True
+    error = None
+    min_confidence = 0.6
+    model_version = "bodytype:vfake"
+
+    def __init__(self, classes: list[str], answer: tuple[str, float] | None) -> None:
+        self.classes = classes
+        self.answer = answer
+        self.calls = 0
+
+    def classify(self, frame, box, plate):  # type: ignore[no-untyped-def]
+        self.calls += 1
+        if isinstance(self.answer, Exception):
+            raise self.answer
+        return self.answer
+
+
+class FrameDetector:
+    """A detector that answers on decoded frames (like YOLOX) with a fixed result."""
+
+    model_version = "det"
+    ready = True
+    error = None
+
+    def __init__(self, result: VehicleResult | None) -> None:
+        self.result = result
+
+    def detect_frame(self, frame, plate):  # type: ignore[no-untyped-def]
+        return self.result
+
+    def detect(self, image_bytes: bytes, plate: BBox | None) -> VehicleResult | None:
+        raise AssertionError("the refined stage should share the decoded frame")
+
+
+def _jpeg() -> bytes:
+    cv2 = pytest.importorskip("cv2")
+    ok, buf = cv2.imencode(".jpg", np.zeros((60, 80, 3), dtype=np.uint8))
+    assert ok
+    return bytes(buf)
+
+
+def test_refined_detector_replaces_car_when_confident_else_keeps_the_detector() -> None:
+    from vision_service.vehicle import RefinedVehicleDetector
+
+    car = VehicleResult(body_type="car", confidence=0.85, bbox=BBox(x1=10, y1=10, x2=70, y2=50))
+    sure = FakeClassifier(["sedan", "suv"], ("suv", 0.91))
+    res = RefinedVehicleDetector(FrameDetector(car), sure).detect(_jpeg(), None)
+    assert (
+        res is not None and res.body_type == "suv" and res.confidence == 0.91 and res.detector_class == "car"
+    )
+    assert res.bbox == car.bbox
+
+    unsure = FakeClassifier(["sedan", "suv"], ("suv", 0.4))
+    res2 = RefinedVehicleDetector(FrameDetector(car), unsure).detect(_jpeg(), None)
+    assert (
+        res2 is not None
+        and res2.body_type == "car"
+        and res2.confidence == 0.85
+        and res2.detector_class == "car"
+    )
+
+    # A class the classifier never trained on is left alone (its softmax means nothing there).
+    bus = VehicleResult(body_type="bus", confidence=0.9, bbox=car.bbox)
+    skip = FakeClassifier(["sedan", "suv"], ("suv", 0.99))
+    res3 = RefinedVehicleDetector(FrameDetector(bus), skip).detect(_jpeg(), None)
+    assert res3 == bus and skip.calls == 0
+    # …unless it was: a classifier that knows trucks may override a truck.
+    knows = FakeClassifier(["sedan", "truck", "van"], ("van", 0.8))
+    truck = VehicleResult(body_type="truck", confidence=0.7, bbox=car.bbox)
+    res4 = RefinedVehicleDetector(FrameDetector(truck), knows).detect(_jpeg(), None)
+    assert res4 is not None and res4.body_type == "van" and res4.detector_class == "truck"
+
+
+def test_refined_detector_survives_a_broken_classifier_and_reports_it() -> None:
+    from vision_service.vehicle import RefinedVehicleDetector
+
+    car = VehicleResult(body_type="car", confidence=0.85, bbox=BBox(x1=10, y1=10, x2=70, y2=50))
+    boom = FakeClassifier(["sedan"], RuntimeError("bad graph"))  # type: ignore[arg-type]
+    ref = RefinedVehicleDetector(FrameDetector(car), boom)
+    assert ref.detect(_jpeg(), None) == car
+    assert ref.error == "classifier: RuntimeError: bad graph"
+    assert ref.ready is True and ref.model_version == "det+bodytype:vfake"
+    # No box, or a detector that found nothing → nothing to classify.
+    assert RefinedVehicleDetector(FrameDetector(None), boom).detect(_jpeg(), None) is None
+    boxless = VehicleResult(body_type="car", confidence=0.85)
+    assert RefinedVehicleDetector(FrameDetector(boxless), boom).detect(_jpeg(), None) == boxless
+
+
+def test_classifier_without_files_is_not_ready_and_the_factory_skips_a_missing_model(tmp_path) -> None:  # type: ignore[no-untyped-def]
+    from vision_service.recognizer import WithVehicle, build_recognizer
+    from vision_service.settings import Settings
+    from vision_service.vehicle import BodyTypeClassifier, RefinedVehicleDetector
+
+    clf = BodyTypeClassifier(str(tmp_path / "bodytype.onnx"))
+    assert clf.ready is False and "FileNotFoundError" in (clf.error or "")
+    (tmp_path / "bodytype.json").write_text('{"format": "other"}')
+    assert "unknown sidecar format" in (BodyTypeClassifier(str(tmp_path / "bodytype.onnx")).error or "")
+
+    # A path with no file = the normal pre-model state: phase A only, no error in health.
+    s = Settings(
+        vehicle_model_path="/nonexistent/yolox.onnx", vehicle_classifier_path=str(tmp_path / "none.onnx")
+    )
+    rec = build_recognizer(s)
+    assert isinstance(rec, WithVehicle)
+    assert not isinstance(rec._detector, RefinedVehicleDetector)  # noqa: SLF001
+    assert "classifier" not in (rec.error or "")