Add end-to-end encryption for chat messages (X25519 / NaCl box)

Messages between matched users are now encrypted client-side with each
user's own key pair before they ever reach the server; the backend only
ever stores/relays opaque ciphertext + nonce and cannot read message
content, including for the last-message preview on the matches list.

- User gains a PublicKey field, registered/rotated via PUT /profile/public-key
- Message gains CipherText/Nonce; legacy plaintext Text is kept as-is for
  messages sent before this change
- MatchView exposes the partner's public key and the last message's
  ciphertext/nonce so the client can decrypt previews itself
- Removed the now-dead plaintext SanitizeMessageText utility — the server
  can no longer sanitize message content since it's opaque ciphertext

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/service/chat_service.go

@@ -1,16 +1,19 @@
 package service
 
 import (
+	"encoding/base64"
 	"errors"
 	"time"
 
 	"dating-app/internal/dto"
 	"dating-app/internal/models"
 	"dating-app/internal/repository"
-	"dating-app/internal/utils"
 )
 
-var ErrNotMatchMember = errors.New("این گفتگو مال تو نیست")
+var (
+	ErrNotMatchMember         = errors.New("این گفتگو مال تو نیست")
+	ErrInvalidMessageEncoding = errors.New("پیام رمزنگاری‌شده نامعتبر است (کدگذاری اشتباه)")
+)
 
 type ChatService struct {
 	matchRepo   *repository.MatchRepository
@@ -85,17 +88,23 @@ func (s *ChatService) ListMatches(userID uint) ([]dto.MatchView, error) {
 		}
 
 		view := dto.MatchView{
-			ID:        m.ID,
-			UserID:    other.ID,
-			FirstName: other.FirstName,
-			Photos:    orEmptyStrings(photosByUser[otherID]),
-			CreatedAt: m.CreatedAt.Format(time.RFC3339),
+			ID:               m.ID,
+			UserID:           other.ID,
+			FirstName:        other.FirstName,
+			Photos:           orEmptyStrings(photosByUser[otherID]),
+			CreatedAt:        m.CreatedAt.Format(time.RFC3339),
+			PartnerPublicKey: other.PublicKey,
 		}
 		if last, ok := lastMessages[m.ID]; ok {
-			text := last.Text
 			at := last.CreatedAt.Format(time.RFC3339)
-			view.LastMessage = &text
 			view.LastAt = &at
+			if last.CipherText != nil {
+				view.LastMessageCipherText = last.CipherText
+				view.LastMessageNonce = last.Nonce
+			} else {
+				text := last.Text
+				view.LastMessage = &text
+			}
 		}
 		views = append(views, view)
 	}
@@ -125,39 +134,46 @@ func (s *ChatService) ListMessages(userID, matchID uint) ([]dto.MessageView, err
 	views := make([]dto.MessageView, 0, len(messages))
 	for _, m := range messages {
 		views = append(views, dto.MessageView{
-			ID:        m.ID,
-			MatchID:   m.MatchID,
-			SenderID:  m.SenderID,
-			Text:      m.Text,
-			CreatedAt: m.CreatedAt.Format(time.RFC3339),
-			IsMine:    m.SenderID == userID,
+			ID:         m.ID,
+			MatchID:    m.MatchID,
+			SenderID:   m.SenderID,
+			Text:       m.Text,
+			CipherText: m.CipherText,
+			Nonce:      m.Nonce,
+			CreatedAt:  m.CreatedAt.Format(time.RFC3339),
+			IsMine:     m.SenderID == userID,
 		})
 	}
 	return views, nil
 }
 
-// SendMessage یک پیام جدید توی یک مچ ثبت می‌کنه؛ فقط اگه کاربر عضو اون مچ باشه.
-func (s *ChatService) SendMessage(userID, matchID uint, text string) (*dto.MessageView, error) {
+// SendMessage یک پیام رمزنگاری‌شده توی یک مچ ثبت می‌کنه؛ فقط اگه کاربر عضو
+// اون مچ باشه. سرور محتوای پیام رو نمی‌بینه و نمی‌تونه — CipherText/Nonce
+// قبلاً روی دستگاه فرستنده با کلید عمومی طرف مقابل رمزنگاری شدن؛ اینجا فقط
+// اندازه‌ی base64 اونا (از طریق binding توی SendMessageRequest) چک می‌شه.
+func (s *ChatService) SendMessage(userID, matchID uint, cipherText, nonce string) (*dto.MessageView, error) {
 	if _, err := s.getAuthorizedMatch(userID, matchID); err != nil {
 		return nil, err
 	}
-
-	clean, err := utils.SanitizeMessageText(text)
-	if err != nil {
-		return nil, err
+	if _, err := base64.StdEncoding.DecodeString(cipherText); err != nil {
+		return nil, ErrInvalidMessageEncoding
+	}
+	if _, err := base64.StdEncoding.DecodeString(nonce); err != nil {
+		return nil, ErrInvalidMessageEncoding
 	}
 
-	msg := &models.Message{MatchID: matchID, SenderID: userID, Text: clean}
+	msg := &models.Message{MatchID: matchID, SenderID: userID, CipherText: &cipherText, Nonce: &nonce}
 	if err := s.messageRepo.Create(msg); err != nil {
 		return nil, err
 	}
 
 	return &dto.MessageView{
-		ID:        msg.ID,
-		MatchID:   msg.MatchID,
-		SenderID:  msg.SenderID,
-		Text:      msg.Text,
-		CreatedAt: msg.CreatedAt.Format(time.RFC3339),
-		IsMine:    true,
+		ID:         msg.ID,
+		MatchID:    msg.MatchID,
+		SenderID:   msg.SenderID,
+		CipherText: msg.CipherText,
+		Nonce:      msg.Nonce,
+		CreatedAt:  msg.CreatedAt.Format(time.RFC3339),
+		IsMine:     true,
 	}, nil
 }