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/ai.py

@@ -1,9 +1,9 @@
 """AI router: model listing and chat completions via OpenRouter."""
 from fastapi import APIRouter, Depends, HTTPException, status
 
-from backend.app.dependencies import get_current_user
-from backend.app.models.ai import ChatRequest, ChatResponse, ModelInfo
-from backend.app.services import openrouter
+from ..dependencies import get_current_user
+from ..models.ai import ChatRequest, ChatResponse, ModelInfo
+from ..services import openrouter
 
 router = APIRouter(prefix="/ai", tags=["ai"])