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.

services/call_persistence.py

@@ -0,0 +1,70 @@
+"""
+Call Persistence — Writes completed calls and their transcript chunks
+to the database when CallManager.end_call() fires.
+"""
+
+import logging
+import uuid
+from datetime import datetime
+
+from db.database import CallRecord, TranscriptChunk, get_session_factory
+from models.call import ActiveCall, CallStatus
+
+logger = logging.getLogger(__name__)
+
+
+async def persist_call_on_end(call: ActiveCall, final_status: CallStatus) -> None:
+    """Insert a CallRecord and any transcript chunks for `call`.
+
+    Wired into CallManager via _on_call_ended in gateway.start().
+    """
+    try:
+        async with get_session_factory()() as session:
+            record = CallRecord(
+                id=call.id,
+                direction=call.direction,
+                remote_number=call.remote_number,
+                status=final_status.value,
+                mode=call.mode.value,
+                intent=call.intent,
+                started_at=call.started_at,
+                ended_at=datetime.now(),
+                duration=int(call.duration),
+                hold_time=int(call.hold_time),
+                device_used=call.device,
+                call_flow_id=call.call_flow_id,
+                classification_timeline=[
+                    {
+                        "timestamp": c.timestamp,
+                        "audio_type": c.audio_type.value,
+                        "confidence": c.confidence,
+                    }
+                    for c in call.classification_history
+                ],
+                metadata_={"services": list(call.services)},
+            )
+            session.add(record)
+
+            # Each transcript chunk gets its own row with a sequence number
+            # so the dashboard can render them in order with click-to-seek.
+            for seq, text in enumerate(call.transcript_chunks):
+                speaker = "unknown"
+                payload = text
+                if ":" in text:
+                    head, rest = text.split(":", 1)
+                    head = head.strip().lower()
+                    if head in {"caller", "agent", "receptionist", "caller_message"}:
+                        speaker = head if head != "caller_message" else "caller"
+                        payload = rest.strip()
+                session.add(TranscriptChunk(
+                    id=f"tc_{uuid.uuid4().hex[:10]}",
+                    call_id=call.id,
+                    seq=seq,
+                    t_offset_ms=0,
+                    speaker=speaker,
+                    text=payload,
+                ))
+
+            await session.commit()
+    except Exception as e:
+        logger.warning(f"Could not persist call {call.id}: {e}")