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>

src/telegram.zig

@@ -14,7 +14,10 @@ pub const VideoNote = struct {
 
 pub const Message = struct {
     message_id: i64,
-    chat: struct { id: i64 },
+    chat: struct {
+        id: i64,
+        @"type": []const u8 = "private",
+    },
     text: ?[]const u8 = null,
     voice: ?Voice = null,
     video_note: ?VideoNote = null,