Try inband DTMF detection for AX83H

Phone negotiates RFC4733 but may not actually send telephone-event
RTP packets. Inband detects DTMF from audio stream directly.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/asterisk/configmap.yaml

@@ -96,7 +96,7 @@ data:
     allow=ulaw
     allow=alaw
     direct_media=no
-    dtmf_mode=auto
+    dtmf_mode=inband
     rtp_symmetric=yes
     force_rport=yes
     rewrite_contact=yes