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

Cargo.toml

@@ -74,6 +74,9 @@ hex = "0.4"
 # CSPRNG for secure token generation
 rand = "0.9"
 
+# serde-big-array for wa-rs storage (large array serialization)
+serde-big-array = { version = "0.5", optional = true }
+
 # Fast mutexes that don't poison on panic
 parking_lot = "0.12"
 
@@ -139,6 +142,15 @@ probe-rs = { version = "0.30", optional = true }
 # PDF extraction for datasheet RAG (optional, enable with --features rag-pdf)
 pdf-extract = { version = "0.10", optional = true }
 
+# WhatsApp Web client (wa-rs) — optional, enable with --features whatsapp-web
+# Uses wa-rs for Bot and Client, wa-rs-core for storage traits, custom rusqlite backend avoids Diesel conflict.
+wa-rs = { version = "0.2", optional = true, default-features = false }
+wa-rs-core = { version = "0.2", optional = true, default-features = false }
+wa-rs-binary = { version = "0.2", optional = true, default-features = false }
+wa-rs-proto = { version = "0.2", optional = true, default-features = false }
+wa-rs-ureq-http = { version = "0.2", optional = true }
+wa-rs-tokio-transport = { version = "0.2", optional = true, default-features = false }
+
 # Raspberry Pi GPIO / Landlock (Linux only) — target-specific to avoid compile failure on macOS
 [target.'cfg(target_os = "linux")'.dependencies]
 rppal = { version = "0.22", optional = true }
@@ -161,6 +173,9 @@ landlock = ["sandbox-landlock"]
 probe = ["dep:probe-rs"]
 # rag-pdf = PDF ingestion for datasheet RAG
 rag-pdf = ["dep:pdf-extract"]
+# whatsapp-web = Native WhatsApp Web client with custom rusqlite storage backend
+whatsapp-web = ["dep:wa-rs", "dep:wa-rs-core", "dep:wa-rs-binary", "dep:wa-rs-proto", "dep:wa-rs-ureq-http", "dep:wa-rs-tokio-transport", "serde-big-array"]
+
 [profile.release]
 opt-level = "z"      # Optimize for size
 lto = "thin"         # Lower memory use during release builds