V1.0: Class 1 fax — real-world 4-page send to external number confirmed

Core features:
- Class 1 T.30 protocol: full send/receive implementation
- HDLC: DLE-stuffing, FCS strip, USR5637 bit-reversal handling
- T.4 MH encoder/decoder (1728px A4 standard)
- Document pipeline: PDF (Ghostscript), PNG, TIFF input
- Width clamping: US Letter 1734px → 1728px fax standard
- Cover page: CJK rasterization (TW/CN/JP/EN), TIFF + HTML output
- OCR verification: Tesseract 5 with eng+chi_tra, CJK space-tolerant
- API server (axum): health, send, jobs, cover, retry, cancel
- Background worker: auto-poll queue, speed fallback, retry policy
- Modem detection, pool management

Real-world test results (2026-07-23):
- V90 → 25153038: 4 pages, V.17 12000 bps, 2:33 ✅
- USR5637 → 25153038: 4 pages, V.17 12000 bps, 2:26 ✅
- Both faxes confirmed received on remote machine

Tested: loopback (100% pixel match), multi-page, all input formats,
cover pages, OCR verify, API endpoints, worker processing.
13 unit tests pass, 0 new clippy warnings.

src/email/smtp.rs

@@ -0,0 +1,111 @@
+use serde::{Deserialize, Serialize};
+use lettre::{
+    message::Message,
+    transport::smtp::authentication::Credentials,
+    SmtpTransport, Transport,
+};
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct EmailConfig {
+    pub smtp_host: String,
+    pub smtp_port: u16,
+    pub smtp_username: String,
+    pub smtp_password: String,
+    pub from_address: String,
+    pub from_name: Option<String>,
+    #[serde(default)]
+    pub use_tls: bool,
+}
+
+pub struct EmailSender {
+    config: EmailConfig,
+}
+
+impl EmailSender {
+    pub fn new(config: EmailConfig) -> Self {
+        Self { config }
+    }
+    
+    pub fn send_email(
+        &self,
+        to: &str,
+        subject: &str,
+        body: &str,
+    ) -> crate::error::Result<()> {
+        let email = Message::builder()
+            .from(
+                format!(
+                    "{} <{}>",
+                    self.config.from_name.as_deref().unwrap_or("Telfax"),
+                    self.config.from_address
+                )
+                .parse()
+                .unwrap(),
+            )
+            .to(to.parse().unwrap())
+            .subject(subject)
+            .body(body.to_string())
+            .unwrap();
+        
+        let creds = Credentials::new(
+            self.config.smtp_username.clone(),
+            self.config.smtp_password.clone(),
+        );
+        
+        let mailer = SmtpTransport::relay(&self.config.smtp_host)
+            .unwrap()
+            .credentials(creds)
+            .port(self.config.smtp_port)
+            .build();
+        
+        mailer.send(&email)
+            .map_err(|e| crate::error::FaxError::Other(format!("Failed to send email: {}", e)))?;
+        
+        Ok(())
+    }
+    
+    pub fn send_success_notification(
+        &self,
+        to: &str,
+        fax_number: &str,
+        job_id: &str,
+    ) -> crate::error::Result<()> {
+        let subject = format!("Fax sent successfully to {}", fax_number);
+        let body = format!(
+            "Your fax to {} has been sent successfully.\n\nJob ID: {}\n\nThis is an automated message from Telfax Fax Server.",
+            fax_number, job_id
+        );
+        
+        self.send_email(to, &subject, &body)
+    }
+    
+    pub fn send_failure_notification(
+        &self,
+        to: &str,
+        fax_number: &str,
+        job_id: &str,
+        error: &str,
+    ) -> crate::error::Result<()> {
+        let subject = format!("Fax failed to {}", fax_number);
+        let body = format!(
+            "Your fax to {} has failed.\n\nJob ID: {}\nError: {}\n\nPlease check the fax number and try again.\n\nThis is an automated message from Telfax Fax Server.",
+            fax_number, job_id, error
+        );
+        
+        self.send_email(to, &subject, &body)
+    }
+    
+    pub fn send_received_notification(
+        &self,
+        to: &str,
+        from_number: &str,
+    ) -> crate::error::Result<()> {
+        let subject = format!("Fax received from {}", from_number);
+        let body = format!(
+            "You have received a fax from {}.\n\nPlease check the fax server for the attachment.\n\nThis is an automated message from Telfax Fax Server.",
+            from_number
+        );
+        
+        self.send_email(to, &subject, &body)
+    }
+}