docs: add deployment validation plan and expand env config

Add comprehensive deployment and validation plan documenting a staged
bring-up approach that gates each layer on its predecessor and defers
PSTN testing until everything else is proven.

Update .env.example to reflect current configuration:
- Replace static API_TOKEN with Casdoor SSO + owner-minted PAT auth
- Add Rhema TTS settings with port-collision warning
- Add AI Receptionist settings for inbound calls
- Reconcile GATEWAY_SIP_PORT to 5060 and default SIP domain

services/audio_classifier.py

@@ -144,7 +144,11 @@ class AudioClassifier:
         )
 
         # 5. If it's speech-like, is it live or automated?
-        if speech_score > music_score:
+        #    A confident music score wins outright: the speech bands are wide
+        #    enough that sustained hold music satisfies all four and scores 1.0,
+        #    which would otherwise mask music as LIVE_HUMAN and ring the owner
+        #    for a hold queue.
+        if speech_score > music_score and music_score < self.settings.music_threshold:
             # Use history to distinguish live human from IVR
             # IVR: repetitive patterns, synthetic prosody
             # Human: natural variation, conversational rhythm