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/call.py

@@ -31,6 +31,7 @@ class CallMode(str, Enum):
     DIRECT = "direct"  # Call and connect immediately
     HOLD_SLAYER = "hold_slayer"  # Navigate IVR, wait on hold, transfer when human
     AI_ASSISTED = "ai_assisted"  # Connect with transcription, recording, noise cancel
+    RECEPTIONIST = "receptionist"  # AI screens inbound caller, then routes or takes a message
 
 
 class AudioClassification(str, Enum):