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/router/router.go

@@ -63,6 +63,7 @@ func Setup(
 		profile.POST("/checkin", h.Profile.CheckIn)
 		profile.PATCH("/notifications", h.Profile.UpdateNotificationSettings)
 		profile.DELETE("/me", h.Profile.DeleteAccount)
+		profile.PUT("/public-key", h.Profile.UpdatePublicKey)
 	}
 
 	// -------- آنبوردینگ (نیاز به ورود) --------