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

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

backend/app/models/ai.py

@@ -74,6 +74,7 @@ class VideoRequest(BaseModel):
     prompt: str
     duration_seconds: int | None = None
     aspect_ratio: str = "16:9"
+    resolution: str | None = None  # e.g. "480p", "720p", "1080p"
 
 
 class VideoFromImageRequest(BaseModel):
@@ -82,6 +83,7 @@ class VideoFromImageRequest(BaseModel):
     prompt: str
     duration_seconds: int | None = None
     aspect_ratio: str = "16:9"
+    resolution: str | None = None  # e.g. "480p", "720p", "1080p"
 
 
 class VideoResponse(BaseModel):