feat(US2): image quad extraction — POST /api/v1/image/extract

- app/models/image_models.py: BBox, QuadrupleItem, ImageExtract{Request,Response}
- app/services/image_service.py: download → base64 LLM → bbox clamp → crop upload
- app/routers/image.py: POST /image/extract handler
- tests: 4 service + 3 router tests, 7/7 passing

tests/test_image_router.py

@@ -0,0 +1,63 @@
+import json
+import numpy as np
+import cv2
+import pytest
+from unittest.mock import AsyncMock
+
+from app.core.exceptions import StorageError
+
+
+def _make_test_image_bytes() -> bytes:
+    img = np.zeros((80, 100, 3), dtype=np.uint8)
+    _, buf = cv2.imencode(".jpg", img)
+    return buf.tobytes()
+
+
+SAMPLE_QUADS_JSON = json.dumps([
+    {
+        "subject": "电缆接头",
+        "predicate": "位于",
+        "object": "配电箱左侧",
+        "qualifier": "2024年检修",
+        "bbox": {"x": 5, "y": 5, "w": 20, "h": 15},
+    }
+])
+
+
+def test_image_extract_returns_200(client, mock_llm, mock_storage):
+    mock_storage.download_bytes = AsyncMock(return_value=_make_test_image_bytes())
+    mock_llm.chat_vision = AsyncMock(return_value=SAMPLE_QUADS_JSON)
+    mock_storage.upload_bytes = AsyncMock(return_value=None)
+
+    resp = client.post(
+        "/api/v1/image/extract",
+        json={"file_path": "image/test.jpg", "task_id": 1},
+    )
+    assert resp.status_code == 200
+    data = resp.json()
+    assert "items" in data
+    assert data["items"][0]["subject"] == "电缆接头"
+    assert data["items"][0]["cropped_image_path"] == "crops/1/0.jpg"
+
+
+def test_image_extract_llm_parse_error_returns_502(client, mock_llm, mock_storage):
+    mock_storage.download_bytes = AsyncMock(return_value=_make_test_image_bytes())
+    mock_llm.chat_vision = AsyncMock(return_value="not json {{")
+
+    resp = client.post(
+        "/api/v1/image/extract",
+        json={"file_path": "image/test.jpg", "task_id": 1},
+    )
+    assert resp.status_code == 502
+    assert resp.json()["code"] == "LLM_PARSE_ERROR"
+
+
+def test_image_extract_storage_error_returns_502(client, mock_storage):
+    mock_storage.download_bytes = AsyncMock(side_effect=StorageError("RustFS down"))
+
+    resp = client.post(
+        "/api/v1/image/extract",
+        json={"file_path": "image/test.jpg", "task_id": 1},
+    )
+    assert resp.status_code == 502
+    assert resp.json()["code"] == "STORAGE_ERROR"