feat(smtp): implement built-in SMTP server for mail ingestion

- Add lightweight SMTP server support using `lettre` and `tokio`.
- Implement `DATA_SMTP_INGEST` permission check for inbound mail.
- Support real-time email archiving via SMTP protocol.
- Integrate with existing EML index manager for automated indexing.

src/modules/envelope/extractor.rs

@@ -61,6 +61,21 @@ pub fn extract_envelope_from_eml(
     )
 }
 
+pub fn extract_envelope_from_smtp(
+    body: &[u8],
+    account_id: u64,
+    mailbox_id: u64,
+) -> BichonResult<(Envelope, Vec<AttachmentInfo>)> {
+    extract_envelope_core(
+        body,
+        0,
+        body.len() as u32,
+        utc_now!(),
+        account_id,
+        mailbox_id,
+    )
+}
+
 fn extract_envelope_core(
     body: &[u8],
     uid: u32,