Refactor import statements across multiple modules to use relative paths for better organization and maintainability.

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

backend/app/routers/generate.py

@@ -1,8 +1,8 @@
 """Generate router: text, image, video, and image-to-video generation."""
 from fastapi import APIRouter, Depends, HTTPException, status
 
-from backend.app.dependencies import get_current_user
-from backend.app.models.ai import (
+from ..dependencies import get_current_user
+from ..models.ai import (
     ImageRequest,
     ImageResponse,
     ImageResult,
@@ -12,7 +12,7 @@ from backend.app.models.ai import (
     VideoRequest,
     VideoResponse,
 )
-from backend.app.services import openrouter
+from ..services import openrouter
 
 router = APIRouter(prefix="/generate", tags=["generate"])