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.

dashboard/src/routes/+layout.svelte

@@ -42,7 +42,11 @@
 		return () => mq.removeEventListener('change', onSystemChange);
 	});
 
-	const nav = [{ href: '/', label: 'Dashboard' }];
+	const nav = [
+		{ href: '/', label: 'Dashboard' },
+		{ href: '/history', label: 'History' },
+		{ href: '/routing', label: 'Routing' },
+	];
 </script>
 
 <div class="min-h-screen bg-slate-50 dark:bg-gray-950 text-gray-900 dark:text-gray-100">