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

config.py

@@ -0,0 +1,119 @@
+"""
+Hold Slayer Gateway — Configuration
+
+All settings loaded from environment variables / .env file.
+"""
+
+from pydantic import Field
+from pydantic_settings import BaseSettings, SettingsConfigDict
+
+
+class SIPTrunkSettings(BaseSettings):
+    """SIP trunk provider configuration."""
+
+    model_config = SettingsConfigDict(env_prefix="SIP_TRUNK_")
+
+    host: str = "sip.provider.com"
+    port: int = 5060
+    username: str = ""
+    password: str = ""
+    transport: str = "udp"  # udp, tcp, tls
+    did: str = ""  # Your phone number (E.164)
+
+
+class GatewaySIPSettings(BaseSettings):
+    """Gateway SIP listener for device registration."""
+
+    model_config = SettingsConfigDict(env_prefix="GATEWAY_SIP_")
+
+    host: str = "0.0.0.0"
+    port: int = 5060
+    domain: str = "gateway.local"
+
+
+class SpeachesSettings(BaseSettings):
+    """Speaches STT service configuration."""
+
+    model_config = SettingsConfigDict(env_prefix="SPEACHES_")
+
+    url: str = "http://localhost:22070"
+    prod_url: str = "http://localhost:22070"
+    model: str = "whisper-large-v3"
+
+
+class ClassifierSettings(BaseSettings):
+    """Audio classifier thresholds."""
+
+    model_config = SettingsConfigDict(env_prefix="CLASSIFIER_")
+
+    music_threshold: float = 0.7
+    speech_threshold: float = 0.6
+    silence_threshold: float = 0.85
+    window_seconds: float = 3.0
+
+
+class LLMSettings(BaseSettings):
+    """LLM service configuration (OpenAI-compatible API)."""
+
+    model_config = SettingsConfigDict(env_prefix="LLM_")
+
+    base_url: str = "http://localhost:11434/v1"
+    model: str = "llama3"
+    api_key: str = "not-needed"
+    timeout: float = 30.0
+    max_tokens: int = 1024
+    temperature: float = 0.3
+
+
+class HoldSlayerSettings(BaseSettings):
+    """Hold Slayer behavior settings."""
+
+    model_config = SettingsConfigDict(env_prefix="HOLD_SLAYER_", env_prefix_allow_empty=True)
+
+    default_transfer_device: str = Field(
+        default="sip_phone", validation_alias="DEFAULT_TRANSFER_DEVICE"
+    )
+    max_hold_time: int = Field(default=7200, validation_alias="MAX_HOLD_TIME")
+    hold_check_interval: float = Field(default=2.0, validation_alias="HOLD_CHECK_INTERVAL")
+
+
+class Settings(BaseSettings):
+    """Root application settings."""
+
+    model_config = SettingsConfigDict(
+        env_file=".env",
+        env_file_encoding="utf-8",
+        extra="ignore",
+    )
+
+    # Database
+    database_url: str = "postgresql+asyncpg://holdslayer:changeme@localhost:5432/holdslayer"
+
+    # Server
+    host: str = "0.0.0.0"
+    port: int = 8000
+    debug: bool = True
+    log_level: str = "info"
+
+    # Notifications
+    notify_sms_number: str = ""
+
+    # Sub-configs
+    sip_trunk: SIPTrunkSettings = Field(default_factory=SIPTrunkSettings)
+    gateway_sip: GatewaySIPSettings = Field(default_factory=GatewaySIPSettings)
+    speaches: SpeachesSettings = Field(default_factory=SpeachesSettings)
+    classifier: ClassifierSettings = Field(default_factory=ClassifierSettings)
+    llm: LLMSettings = Field(default_factory=LLMSettings)
+    hold_slayer: HoldSlayerSettings = Field(default_factory=HoldSlayerSettings)
+
+
+# Singleton
+_settings: Settings | None = None
+
+
+def get_settings() -> Settings:
+    """Get cached application settings."""
+    global _settings
+    if _settings is None:
+        _settings = Settings()
+    return _settings