refactor(voice-call): centralize Telnyx webhook verification

docs/plugins/voice-call.md

@@ -70,6 +70,14 @@ Set config under `plugins.entries.voice-call.config`:
             authToken: "...",
           },
 
+          telnyx: {
+            apiKey: "...",
+            connectionId: "...",
+            // Telnyx webhook public key from the Telnyx Mission Control Portal
+            // (Base64 string; can also be set via TELNYX_PUBLIC_KEY).
+            publicKey: "...",
+          },
+
           plivo: {
             authId: "MAxxxxxxxxxxxxxxxxxxxx",
             authToken: "...",