feat(mail): wire resolveThreadID into processMessage

- Add references variable to extract References header
- Replace manual inReplyTo handling with resolveThreadID call
- Maintain existing body parsing functionality
- Includes proper lambda function for database lookups

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

internal/mail/imap.go

@@ -366,6 +366,7 @@ func (w *IMAPWatcher) processMessage(c *imapclient.Client, buf *imapclient.Fetch
 	// 4. Threading: determine thread_id.
 	threadID := messageID
 	inReplyTo := ""
+	references := ""
 
 	// Parse body to get In-Reply-To / References headers and plain text.
 	bodyPlain := ""
@@ -373,10 +374,12 @@ func (w *IMAPWatcher) processMessage(c *imapclient.Client, buf *imapclient.Fetch
 	if rawBody != nil {
 		mr, err := gomessage.CreateReader(io.NopCloser(strings.NewReader(string(rawBody))))
 		if err == nil {
-			// Extract In-Reply-To header.
 			if irt, err := mr.Header.Text("In-Reply-To"); err == nil && irt != "" {
 				inReplyTo = strings.TrimSpace(irt)
 			}
+			if refs, err := mr.Header.Text("References"); err == nil && refs != "" {
+				references = refs
+			}
 
 			// Read body parts for plain text.
 			for {
@@ -394,12 +397,9 @@ func (w *IMAPWatcher) processMessage(c *imapclient.Client, buf *imapclient.Fetch
 		}
 	}
 
-	if inReplyTo != "" {
-		parent, err := database.FindByMessageID(w.db, inReplyTo)
-		if err == nil && parent != nil {
-			threadID = parent.ThreadID
-		}
-	}
+	threadID = resolveThreadID(messageID, inReplyTo, references, func(mid string) (*database.Task, error) {
+		return database.FindByMessageID(w.db, mid)
+	})
 
 	// 5. Save task with status RECEIVED.
 	taskUUID := uuid.New().String()