feat: add call history API endpoints and TTS service client

Adds read-only access to persisted call records for the dashboard
and implements a client for the Rhema text-to-speech service.

- api/call_history.py: New router providing paged call lists
  and detailed call records with transcript metadata.
- services/tts.py: Async client for OpenAI-compatible TTS
  endpoints (Rhema/Kokoro) used for call-flow steps.

models/device.py

@@ -38,6 +38,7 @@ class Device(DeviceBase):
 
     id: str
     is_online: bool = False
+    dnd: bool = False
     last_seen: Optional[datetime] = None
     created_at: Optional[datetime] = None
     updated_at: Optional[datetime] = None
@@ -45,6 +46,8 @@ class Device(DeviceBase):
     @property
     def can_receive_call(self) -> bool:
         """Can this device receive a call right now?"""
+        if self.dnd:
+            return False
         if self.type in (DeviceType.SIP_PHONE, DeviceType.SOFTPHONE, DeviceType.WEBRTC):
             return self.is_online and self.sip_uri is not None
         if self.type == DeviceType.CELL: