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/models/image_models.py

@@ -0,0 +1,28 @@
+from pydantic import BaseModel
+
+
+class BBox(BaseModel):
+    x: int
+    y: int
+    w: int
+    h: int
+
+
+class QuadrupleItem(BaseModel):
+    subject: str
+    predicate: str
+    object: str
+    qualifier: str | None = None
+    bbox: BBox
+    cropped_image_path: str
+
+
+class ImageExtractRequest(BaseModel):
+    file_path: str
+    task_id: int
+    model: str | None = None
+    prompt_template: str | None = None
+
+
+class ImageExtractResponse(BaseModel):
+    items: list[QuadrupleItem]