Implement a webhook dispatcher in the Rust service

[mftee]

Implement a webhook dispatcher in the Rust service that emits HTTP POST callbacks to registered URLs when significant verification events occur (hash submitted, hash verified, document revoked).

Requirements:

• Create a WebhookConfig struct read from environment:

  pub struct WebhookConfig {
      pub urls: Vec<String>,       // comma-separated WEBHOOK_URLS env var
      pub secret: String,          // WEBHOOK_SECRET env var
      pub timeout_secs: u64,       // WEBHOOK_TIMEOUT_SECS, default 5
  }

• Create a WebhookDispatcher with:
  • dispatch(event_type: &str, payload: serde_json::Value) -> Vec<DeliveryResult> — POSTs the payload to all configured URLs concurrently
  • Sign the request body with HMAC-SHA256 using secret and include as X-SMALDA-Signature
  • Retry failed deliveries once after a 1-second delay
• DeliveryResult:

  pub struct DeliveryResult {
      pub url: String,
      pub success: bool,
      pub status_code: Option<u16>,
      pub error: Option<String>,
  }

• Fire webhooks asynchronously using tokio::spawn so they do not block the response

Acceptance Criteria:

• Dispatching is non-blocking — the main request response is not delayed
• HMAC signature is computed correctly and verifiable by the receiver
• Unreachable URLs are logged but do not panic
• Unit tests mock HTTP calls and verify HMAC signature computation