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

app/routers/image.py

@@ -1,3 +1,18 @@
-from fastapi import APIRouter
+from fastapi import APIRouter, Depends
+
+from app.clients.llm.base import LLMClient
+from app.clients.storage.base import StorageClient
+from app.core.dependencies import get_llm_client, get_storage_client
+from app.models.image_models import ImageExtractRequest, ImageExtractResponse
+from app.services import image_service
 
 router = APIRouter(tags=["Image"])
+
+
+@router.post("/image/extract", response_model=ImageExtractResponse)
+async def extract_image(
+    req: ImageExtractRequest,
+    llm: LLMClient = Depends(get_llm_client),
+    storage: StorageClient = Depends(get_storage_client),
+) -> ImageExtractResponse:
+    return await image_service.extract_quads(req, llm, storage)