fix: remove unused import and hoist module-level constant

- Remove unused  from tools/tts_tool.py (dead code)
- Move _BUILTIN_DELIVER_PLATFORMS set from send() method to module
  scope in gateway/platforms/webhook.py to avoid reallocation on
  every call

tools/tts_tool.py

@@ -44,7 +44,6 @@ import queue
 import re
 import shlex
 import shutil
-import signal
 import subprocess
 import tempfile
 import threading