fixes generated image download

backend/app/auth.py

@@ -37,6 +37,31 @@ def create_access_token(user_id: int, is_admin: bool) -> str:
     return jwt.encode(payload, settings.JWT_SECRET_KEY, algorithm="HS256")
 
 
+def create_download_token(artifact_id: int) -> str:
+    """Create a short-lived JWT for downloading a specific artifact."""
+    expire = datetime.now(timezone.utc) + timedelta(minutes=settings.DOWNLOAD_TOKEN_MINUTES)
+    payload = {
+        "sub": "download",
+        "artifact_id": artifact_id,
+        "exp": expire,
+    }
+    return jwt.encode(payload, settings.JWT_SECRET_KEY, algorithm="HS256")
+
+
+def verify_download_token(token: str) -> int | None:
+    """Verify a download token and return the artifact_id, or None on failure."""
+    try:
+        payload = jwt.decode(token, settings.JWT_SECRET_KEY, algorithms=["HS256"])
+        if payload.get("sub") != "download":
+            return None
+        artifact_id = payload.get("artifact_id")
+        if not isinstance(artifact_id, int):
+            return None
+        return artifact_id
+    except JWTError:
+        return None
+
+
 def decode_token(token: str) -> dict:
     """Decode and verify a JWT token. Raises HTTPException 401 on failure."""
     try: