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/src/storage/twilio-storage.ts

@@ -24,6 +24,9 @@ export type CommunicationCreateData = {
   body?: string;
   callDuration?: number;
   twilioSid?: string;
+  // Which platform produced twilioSid — defaults to "twilio" (dial-pad calls) when
+  // omitted; Telnyx call sites (everything else) pass "telnyx" explicitly.
+  provider?: "twilio" | "telnyx";
 };
 
 export const twilioStorage = {