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/utils/sanitize.go

@@ -1,47 +0,0 @@
-package utils
-
-import (
-	"errors"
-	"strings"
-	"unicode/utf8"
-)
-
-var (
-	ErrInvalidMessageEncoding = errors.New("متن پیام معتبر نیست (کدگذاری اشتباه)")
-	ErrMessageEmpty           = errors.New("متن پیام نمی‌تواند خالی باشد")
-	ErrMessageTooLong         = errors.New("متن پیام بیش از حد مجاز طولانی است")
-)
-
-const MaxMessageLength = 2000
-
-// SanitizeMessageText متن یک پیام چت رو قبل از ذخیره‌سازی پاک‌سازی/اعتبارسنجی می‌کنه:
-//   - رد کردن رشته‌هایی که UTF-8 معتبر نیستن (بایت خراب/دستکاری‌شده)
-//   - حذف کاراکترهای کنترلی (به‌جز \n و \t) که می‌تونن نمایش یا لاگ‌ها رو خراب کنن
-//   - trim فاصله‌های اضافه و محدود کردن طول
-func SanitizeMessageText(raw string) (string, error) {
-	if !utf8.ValidString(raw) {
-		return "", ErrInvalidMessageEncoding
-	}
-
-	var b strings.Builder
-	b.Grow(len(raw))
-	for _, r := range raw {
-		if r == '\n' || r == '\t' {
-			b.WriteRune(r)
-			continue
-		}
-		if r < 0x20 || r == 0x7F {
-			continue
-		}
-		b.WriteRune(r)
-	}
-
-	text := strings.TrimSpace(b.String())
-	if text == "" {
-		return "", ErrMessageEmpty
-	}
-	if utf8.RuneCountInString(text) > MaxMessageLength {
-		return "", ErrMessageTooLong
-	}
-	return text, nil
-}

internal/utils/sanitize_test.go

@@ -1,71 +0,0 @@
-package utils
-
-import (
-	"errors"
-	"strings"
-	"testing"
-)
-
-func TestSanitizeMessageText_TrimsWhitespace(t *testing.T) {
-	got, err := SanitizeMessageText("  سلام دوست من  ")
-	if err != nil {
-		t.Fatalf("unexpected error: %v", err)
-	}
-	if got != "سلام دوست من" {
-		t.Fatalf("expected trimmed text, got %q", got)
-	}
-}
-
-func TestSanitizeMessageText_EmptyAfterTrim(t *testing.T) {
-	_, err := SanitizeMessageText("     ")
-	if !errors.Is(err, ErrMessageEmpty) {
-		t.Fatalf("expected ErrMessageEmpty, got %v", err)
-	}
-}
-
-func TestSanitizeMessageText_StripsControlChars(t *testing.T) {
-	got, err := SanitizeMessageText("سلام\x00\x07دنیا")
-	if err != nil {
-		t.Fatalf("unexpected error: %v", err)
-	}
-	if got != "سلامدنیا" {
-		t.Fatalf("expected control chars stripped, got %q", got)
-	}
-}
-
-func TestSanitizeMessageText_KeepsNewlinesAndTabs(t *testing.T) {
-	got, err := SanitizeMessageText("خط اول\nخط دوم\tبا تب")
-	if err != nil {
-		t.Fatalf("unexpected error: %v", err)
-	}
-	if got != "خط اول\nخط دوم\tبا تب" {
-		t.Fatalf("expected newline/tab preserved, got %q", got)
-	}
-}
-
-func TestSanitizeMessageText_InvalidUTF8(t *testing.T) {
-	invalid := string([]byte{0xff, 0xfe, 0xfd})
-	_, err := SanitizeMessageText(invalid)
-	if !errors.Is(err, ErrInvalidMessageEncoding) {
-		t.Fatalf("expected ErrInvalidMessageEncoding, got %v", err)
-	}
-}
-
-func TestSanitizeMessageText_TooLong(t *testing.T) {
-	long := strings.Repeat("a", MaxMessageLength+1)
-	_, err := SanitizeMessageText(long)
-	if !errors.Is(err, ErrMessageTooLong) {
-		t.Fatalf("expected ErrMessageTooLong, got %v", err)
-	}
-}
-
-func TestSanitizeMessageText_ExactlyMaxLength(t *testing.T) {
-	exact := strings.Repeat("a", MaxMessageLength)
-	got, err := SanitizeMessageText(exact)
-	if err != nil {
-		t.Fatalf("unexpected error at exact max length: %v", err)
-	}
-	if len(got) != MaxMessageLength {
-		t.Fatalf("expected length %d, got %d", MaxMessageLength, len(got))
-	}
-}