fix(account): Telegram display-name falls back to the @username verbatim

When the Telegram first name yields no usable letters, fall back to the @username
taken whole (trimmed + length-capped, never character-stripped like the real name)
rather than a sanitized form; the generated placeholder is reached only when no
username is set. Precedence: real name -> @username (verbatim) -> placeholder.

backend/internal/account/provision_test.go

@@ -9,8 +9,9 @@ import (
 
 // TestTelegramSeed covers the pure mapping from Telegram launch fields to the
 // create-time account seed: supported-language detection (bare and region-tagged),
-// the first-name / username display-name precedence, and the sanitization that
-// strips disallowed characters (emoji, digits, punctuation) to the editable format.
+// the real-name → @username (verbatim) → placeholder display-name precedence, and
+// the sanitization of the real name (emoji, digits, punctuation stripped to the
+// editable format). The username, when used, is kept verbatim.
 func TestTelegramSeed(t *testing.T) {
 	cases := map[string]struct {
 		languageCode, username, firstName string
@@ -28,6 +29,7 @@ func TestTelegramSeed(t *testing.T) {
 		"punct to space":      {"en", "user", "John❤Doe", "en", "John Doe"},
 		"digits dropped":      {"ru", "user", "Маша123", "ru", "Маша"},
 		"garbage to username": {"en", "good", "123!@#", "en", "good"},
+		"username verbatim":   {"en", "co_ol99", "🎮🎮", "en", "co_ol99"},
 	}
 	for name, tc := range cases {
 		t.Run(name, func(t *testing.T) {
@@ -49,10 +51,10 @@ func TestTelegramSeedPlaceholder(t *testing.T) {
 		languageCode, username, firstName string
 		wantRe                            string
 	}{
-		"en empty":     {"en", "", "", `^Player-\d{5}$`},
-		"ru empty":     {"ru", "", "", `^Игрок-\d{5}$`},
-		"default en":   {"fr", "", "", `^Player-\d{5}$`},
-		"both garbage": {"ru", "123", "!!!", `^Игрок-\d{5}$`},
+		"en empty":                  {"en", "", "", `^Player-\d{5}$`},
+		"ru empty":                  {"ru", "", "", `^Игрок-\d{5}$`},
+		"default en":                {"fr", "", "", `^Player-\d{5}$`},
+		"name garbage, no username": {"ru", "", "!!!", `^Игрок-\d{5}$`},
 	}
 	for name, tc := range cases {
 		t.Run(name, func(t *testing.T) {