feat(telegram): deny bot users at initData validation

The validator parsed only id/username/first_name/language_code from the signed
Telegram user, so a WebAppUser flagged is_bot would have been provisioned a normal
account. The HMAC already proves Telegram signed the payload, so is_bot==true is
Telegram itself attesting the launching principal is a bot.

Parse is_bot and reject it (ErrInvalidInitData -> gateway 4xx -> launch error). A
real user opening the Mini App never carries it, so this is a defensive deny. Tests
cover both the denied (is_bot true) and allowed (is_bot false) paths.

platform/telegram/internal/initdata/validator.go

@@ -18,7 +18,8 @@ import (
 )
 
 // ErrInvalidInitData is returned when initData fails HMAC validation, is missing
-// the hash, is malformed, or is older than the freshness window.
+// the hash, is malformed, is older than the freshness window, or identifies a bot
+// user (is_bot), which is denied.
 var ErrInvalidInitData = errors.New("initdata: invalid telegram init data")
 
 // defaultMaxAge bounds how old a validated initData payload may be.
@@ -120,6 +121,7 @@ func parseUser(userJSON string) (User, error) {
 	}
 	var u struct {
 		ID           int64  `json:"id"`
+		IsBot        bool   `json:"is_bot"`
 		Username     string `json:"username"`
 		FirstName    string `json:"first_name"`
 		LanguageCode string `json:"language_code"`
@@ -127,6 +129,12 @@ func parseUser(userJSON string) (User, error) {
 	if err := json.Unmarshal([]byte(userJSON), &u); err != nil || u.ID == 0 {
 		return User{}, ErrInvalidInitData
 	}
+	// Deny bot principals: the HMAC has already proved Telegram signed this payload, so is_bot==true
+	// is Telegram itself attesting the launching user is a bot. A real user opening the Mini App
+	// never carries it, so reject defensively rather than provision an account for a bot.
+	if u.IsBot {
+		return User{}, ErrInvalidInitData
+	}
 	return User{
 		ExternalID:   strconv.FormatInt(u.ID, 10),
 		Username:     u.Username,

platform/telegram/internal/initdata/validator_test.go

@@ -83,3 +83,28 @@ func TestValidateRejects(t *testing.T) {
 		}
 	})
 }
+
+func TestValidateBotUser(t *testing.T) {
+	t.Run("is_bot true is denied", func(t *testing.T) {
+		initData := signInitData(testToken, map[string]string{
+			"auth_date": strconv.FormatInt(time.Now().Unix(), 10),
+			"user":      `{"id":42,"is_bot":true,"first_name":"Robo"}`,
+		})
+		if _, err := NewHMACValidator(testToken).Validate(initData); !errors.Is(err, ErrInvalidInitData) {
+			t.Errorf("err = %v, want ErrInvalidInitData", err)
+		}
+	})
+	t.Run("is_bot false is allowed", func(t *testing.T) {
+		initData := signInitData(testToken, map[string]string{
+			"auth_date": strconv.FormatInt(time.Now().Unix(), 10),
+			"user":      `{"id":42,"is_bot":false,"first_name":"Thomas"}`,
+		})
+		u, err := NewHMACValidator(testToken).Validate(initData)
+		if err != nil {
+			t.Fatalf("validate: %v", err)
+		}
+		if u.ExternalID != "42" || u.FirstName != "Thomas" {
+			t.Errorf("user = %+v, want {42 Thomas}", u)
+		}
+	})
+}