fix(vision): CI has no numpy — the vehicle-stage tests need it without the alpr extra

CI syncs the service with `uv sync --frozen` (no extra), so test_vehicle.py's module-level
numpy import broke collection in ci.yml and both build-images runs. numpy joins the dev
group (the service imports it lazily); the one test that resizes with OpenCV skips when
cv2 is absent. Reproduced locally in a CI-identical env: 13 passed, 1 skipped; ruff + mypy clean.

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

apps/vision/tests/test_vehicle.py

@@ -6,6 +6,7 @@ from __future__ import annotations
 import os
 
 import numpy as np
+import pytest
 from fastapi.testclient import TestClient
 
 from vision_service.schemas import BBox, VehicleResult
@@ -82,6 +83,7 @@ def test_pick_prefers_the_box_holding_the_plate_else_the_largest() -> None:
 
 
 def test_letterbox_keeps_aspect_and_pads_with_114() -> None:
+    pytest.importorskip("cv2")  # letterbox resizes with OpenCV — only present with the alpr extra
     frame = np.zeros((30, 60, 3), dtype=np.uint8)
     tensor, scale = letterbox(frame, 64)
     assert tensor.shape == (1, 3, 64, 64) and tensor.dtype == np.float32