Add /speak command for multi-voice TTS in group chats

- /speak <voice> enables TTS with selected voice (irina, denis, dmitri)
- /speak stop disables TTS in the chat
- Private chats keep always-on TTS behavior
- Add PIPER_VOICES env var for voice-to-URL mapping
- Add chat type field to distinguish private/group chats

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

k8s/transcribator.yaml

@@ -42,6 +42,8 @@ spec:
               value: "ru"
             - name: PIPER_URL
               value: "http://piper.piper.svc:5000"
+            - name: PIPER_VOICES
+              value: "irina=http://piper.piper.svc:5000,denis=http://piper-denis.piper.svc:5000,dmitri=http://piper-dmitri.piper.svc:5000"
           resources:
             requests:
               memory: "32Mi"