Antispam: merge the duplicated sdk import

Two separate `from app.plugins.sdk import` lines tripped ruff's import sort
under backend's config but not the repo root's — only backend/ has a ruff
config, so plugins/ are checked with different rules. Merged; the underlying
config gap remains worth closing with a root ruff.toml.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018SqJYt7jD7oUEkoY68sBZo

plugins/webmail_plugin_antispam/webmail_plugin_antispam/__init__.py

@@ -18,8 +18,7 @@ from __future__ import annotations
 
 from pathlib import Path
 
-from app.plugins.sdk import Plugin, Slot, UiManifest
-from app.plugins.sdk import JobSpec
+from app.plugins.sdk import JobSpec, Plugin, Slot, UiManifest
 from webmail_plugin_antispam.hook import (
     on_account_state_changed,
     on_message_changed,