feat: add initial Hold Slayer AI telephony gateway implementation

Complete project scaffolding and core implementation of an AI-powered
telephony system that calls companies, navigates IVR menus, waits on
hold, and transfers to the user when a human answers.

Key components:
- FastAPI server with REST API, WebSocket, and MCP (SSE) interfaces
- SIP/VoIP call management via PJSUA2 with RTP audio streaming
- LLM-powered IVR navigation using OpenAI/Anthropic with tool calling
- Hold detection service combining audio analysis and silence detection
- Real-time STT (Whisper/Deepgram) and TTS (OpenAI/Piper) pipelines
- Call recording with per-channel and mixed audio capture
- Event bus (asyncio pub/sub) for real-time client updates
- Web dashboard with live call monitoring
- SQLite persistence via SQLAlchemy with call history and analytics
- Notification support (email, SMS, webhook, desktop)
- Docker Compose deployment with Opal VoIP and Opal Media containers
- Comprehensive test suite with unit, integration, and E2E tests
- Simplified .gitignore and full project documentation in README

api/deps.py

@@ -0,0 +1,17 @@
+"""
+API Dependencies — Shared dependency injection for all routes.
+"""
+
+from fastapi import Depends, HTTPException, Request
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from core.gateway import AIPSTNGateway
+from db.database import get_db
+
+
+def get_gateway(request: Request) -> AIPSTNGateway:
+    """Get the gateway instance from app state."""
+    gateway = getattr(request.app.state, "gateway", None)
+    if gateway is None:
+        raise HTTPException(status_code=503, detail="Gateway not initialized")
+    return gateway