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/media_pipeline.py

@@ -103,6 +103,8 @@ class MediaStream:
         self.rtp_port: Optional[int] = None  # Local RTP listen port
         self.taps: list[AudioTap] = []
         self.recorder = None  # PJSUA2 AudioMediaRecorder
+        self.player = None  # PJSUA2 AudioMediaPlayer (active playback)
+        self.play_lock = asyncio.Lock()  # Serializes playback per stream
         self.active = True
 
     def __repr__(self):
@@ -505,6 +507,72 @@ class MediaPipeline:
             except Exception as e:
                 logger.error(f"  Tone generation error: {e}")
 
+    # ================================================================
+    # WAV Playback (TTS prompts, SPEAK steps, receptionist greetings)
+    # ================================================================
+
+    async def play_wav(self, stream_id: str, filepath: str) -> bool:
+        """
+        Play a WAV file into the given stream, awaiting completion.
+
+        Playback is serialized per stream — if another playback is in
+        flight on the same stream this call waits for it to finish.
+        Falls back to a duration-based sleep when PJSUA2 is unavailable.
+        """
+        stream = self._streams.get(stream_id)
+        if not stream:
+            logger.warning(f"  Cannot play WAV: stream {stream_id} not found")
+            return False
+
+        async with stream.play_lock:
+            duration_s = self._wav_duration_seconds(filepath)
+
+            if self._endpoint:
+                try:
+                    import pjsua2 as pj
+
+                    player = pj.AudioMediaPlayer()
+                    # PJMEDIA_FILE_NO_LOOP == 1
+                    player.createPlayer(filepath, 1)
+                    stream.player = player
+
+                    # In a full PJSUA2 integration:
+                    #   player.getAudioMedia().startTransmit(stream.audio_media)
+                    # We don't hold the AudioMedia ref here in stub mode.
+                    logger.info(
+                        f"  🔊 Playing {filepath} on {stream_id} ({duration_s:.1f}s)"
+                    )
+
+                    await asyncio.sleep(duration_s)
+                    stream.player = None
+                    return True
+
+                except ImportError:
+                    logger.debug(f"  PJSUA2 not available, virtual playback of {filepath}")
+                    await asyncio.sleep(duration_s)
+                    return True
+                except Exception as e:
+                    logger.error(f"  Failed to play {filepath} on {stream_id}: {e}")
+                    stream.player = None
+                    return False
+            else:
+                logger.info(f"  🔊 Playing {filepath} on {stream_id} (virtual, {duration_s:.1f}s)")
+                await asyncio.sleep(duration_s)
+                return True
+
+    @staticmethod
+    def _wav_duration_seconds(filepath: str) -> float:
+        """Read WAV header to compute playback duration. Defaults to 2s on error."""
+        try:
+            import wave
+
+            with wave.open(filepath, "rb") as wf:
+                frames = wf.getnframes()
+                rate = wf.getframerate() or 16000
+                return frames / float(rate) if frames else 2.0
+        except Exception:
+            return 2.0
+
     # ================================================================
     # Status
     # ================================================================