feat(channels): implement WhatsApp Web channel with wa-rs integration

- Add wa-rs dependencies with custom rusqlite storage backend
- Implement functional WhatsApp Web channel using wa-rs Bot
- Integrate TokioWebSocketTransportFactory and UreqHttpClient
- Add message handling via Bot event loop with proper shutdown
- Create WhatsApp storage trait implementations for wa-rs
- Add WhatsApp config schema and onboarding support
- Implement Meta webhook verification for WhatsApp Cloud API
- Add webhook signature verification for security
- Generate unique message keys for WhatsApp conversations
- Remove unused Node.js whatsapp-web-bridge stub

Supersedes: baileys-based bridge approach in favor of native Rust wa-rs

src/channels/whatsapp.rs

@@ -15,6 +15,11 @@ fn ensure_https(url: &str) -> anyhow::Result<()> {
     Ok(())
 }
 
+///
+/// # Runtime Negotiation
+///
+/// This Cloud API channel is automatically selected when `phone_number_id` is set in the config.
+/// Use `WhatsAppWebChannel` (with `session_path`) for native Web mode.
 pub struct WhatsAppChannel {
     access_token: String,
     endpoint_id: String,