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/dto/profile_dto.go

@@ -23,3 +23,10 @@ type UpdateNotificationSettingsRequest struct {
 	NotifyMessages bool `json:"notify_messages"`
 	NotifyLikes    bool `json:"notify_likes"`
 }
+
+// UpdatePublicKeyRequest ثبت/به‌روزرسانی کلید عمومی X25519 کاربر برای
+// رمزنگاری سرتاسری پیام‌ها. کلید باید دقیقاً ۳۲ بایت خام باشه که به‌صورت
+// base64 استاندارد (۴۴ کاراکتر، شامل padding) کدگذاری شده.
+type UpdatePublicKeyRequest struct {
+	PublicKey string `json:"public_key" binding:"required,len=44"`
+}