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.

core/call_manager.py

@@ -30,6 +30,7 @@ class CallManager:
         self.event_bus = event_bus
         self._active_calls: dict[str, ActiveCall] = {}
         self._call_legs: dict[str, str] = {}  # SIP leg ID -> call ID mapping
+        self._on_call_ended = None  # async callback(call: ActiveCall, final_status)
 
     # ================================================================
     # Call Lifecycle
@@ -164,6 +165,11 @@ class CallManager:
                 },
                 message=f"📵 Call ended: {call.remote_number} ({call.duration}s, hold: {call.hold_time}s)",
             ))
+            if self._on_call_ended is not None:
+                try:
+                    await self._on_call_ended(call, status)
+                except Exception as e:
+                    logger.warning(f"on_call_ended hook failed for {call_id}: {e}")
         return call
 
     # ================================================================