fix: resolve 57 ruff lint errors (unused imports, unused variables, import placement)

backend/ai.py

@@ -5,7 +5,6 @@ Configured via environment variables.
 """
 
 import os
-import json
 import logging
 import httpx
 from typing import Optional, Literal

backend/ai_routes.py

@@ -1,6 +1,6 @@
 """ObsiGate AI — API routes for AI-powered editor features."""
 
-from fastapi import APIRouter, HTTPException, Query
+from fastapi import APIRouter, HTTPException
 from pydantic import BaseModel, Field
 from typing import Optional
 import logging

backend/watcher.py

@@ -1,6 +1,5 @@
 import asyncio
 import logging
-import os
 import time
 from pathlib import Path
 from typing import Callable, Dict, List, Optional