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>

nginx.conf

@@ -45,11 +45,12 @@ server {
     }
 }
 
-# ── Public Twilio webhooks only ──────────────────────────────────────────
+# ── Public Twilio/Telnyx webhooks only ───────────────────────────────────
 # https://summit.mydentalofficemanagement.com, reached via Cloudflare Tunnel.
 # Cloudflare terminates TLS at its edge and cloudflared forwards plain HTTP
 # to this block, so no certificate is needed here. Nothing except the
-# Twilio webhook path is exposed on this hostname.
+# Twilio (dial pad only) and Telnyx (everything else) webhook paths is
+# exposed on this hostname.
 server {
     listen 80;
     server_name summit.mydentalofficemanagement.com;
@@ -64,6 +65,14 @@ server {
         proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
     }
 
+    location /api/telnyx/ {
+        proxy_pass http://127.0.0.1:5000;
+        proxy_http_version 1.1;
+        proxy_set_header Host $host;
+        proxy_set_header X-Real-IP $remote_addr;
+        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+    }
+
     location / {
         return 403;
     }