feat(proxy-engine): add on-demand TTS caching for voicemail and IVR prompts

rust/crates/proxy-engine/src/call.rs

@@ -23,6 +23,7 @@ pub enum CallState {
     Ringing,
     Connected,
     Voicemail,
+    Ivr,
     Terminated,
 }
 
@@ -37,6 +38,7 @@ impl CallState {
             Self::Ringing => "ringing",
             Self::Connected => "connected",
             Self::Voicemail => "voicemail",
+            Self::Ivr => "ivr",
             Self::Terminated => "terminated",
         }
     }