Add resolution parameter to video generation requests and implement video status polling endpoint

Co-authored-by: Copilot <copilot@github.com>

backend/app/routers/generate.py

@@ -100,6 +100,7 @@ async def generate_video(
             prompt=body.prompt,
             duration_seconds=body.duration_seconds,
             aspect_ratio=body.aspect_ratio,
+            resolution=body.resolution,
         )
     except Exception as exc:
         raise HTTPException(
@@ -131,6 +132,7 @@ async def generate_video_from_image(
             prompt=body.prompt,
             duration_seconds=body.duration_seconds,
             aspect_ratio=body.aspect_ratio,
+            resolution=body.resolution,
         )
     except Exception as exc:
         raise HTTPException(