feat: migrate SMS/voice to Telnyx (TeXML), keep browser dial pad on Twilio

Twilio now handles only the in-browser dial pad (Access Token + browser-Device
outbound TwiML); Telnyx takes over SMS, inbound voicemail, the AI voice
conversation loop, and staff-triggered outbound calls via TeXML. Adds
TelnyxSettings model, Communication.provider discriminator, new comms/routes
modules, and a real Telnyx Settings page.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/Backend/package.json

@@ -35,6 +35,7 @@
     "pdfkit": "^0.17.2",
     "sharp": "^0.35.3",
     "socket.io": "^4.8.1",
+    "telnyx": "^7.20.0",
     "twilio": "^6.0.0",
     "ws": "^8.18.0",
     "zod": "^3.24.2",