feat(admin): add super admin initialization endpoint

Add POST /api/v1/admin/setup-super-admin endpoint to initialize the first super admin.
The endpoint can only be used once - after a super admin exists, subsequent requests
are rejected. Requires valid setup_secret configured via APP_SETUP_SECRET environment
variable or setup_secret in config file.

internal/pkg/response/response.go

@@ -161,7 +161,8 @@ func statusCodeFromAppErrorCode(code string) int {
 		return http.StatusBadRequest
 	case "FORBIDDEN", "NOT_GROUP_MEMBER", "NOT_GROUP_ADMIN", "NOT_GROUP_OWNER",
 		"USER_BANNED", "USER_BLOCKED", "MUTED", "MUTE_ALL_ENABLED",
-		"SCHEDULE_FORBIDDEN", "UNAUTHORIZED_NOTIFICATION":
+		"SCHEDULE_FORBIDDEN", "UNAUTHORIZED_NOTIFICATION",
+		"SETUP_ALREADY_COMPLETED":
 		return http.StatusForbidden
 	case "BAD_REQUEST", "INVALID_USERNAME", "INVALID_EMAIL", "INVALID_PHONE",
 		"WEAK_PASSWORD", "USERNAME_EXISTS", "EMAIL_EXISTS", "PHONE_EXISTS",
@@ -171,7 +172,8 @@ func statusCodeFromAppErrorCode(code string) int {
 		"CANNOT_JOIN", "JOIN_REQUEST_PENDING", "GROUP_REQUEST_HANDLED",
 		"NOT_REQUEST_TARGET", "NO_ELIGIBLE_INVITEE", "NOT_MUTUAL_FOLLOW",
 		"INVITE_NOT_ACCEPTED", "STICKER_ALREADY_EXISTS", "INVALID_STICKER_URL",
-		"INVALID_SCHEDULE_PAYLOAD", "SCHEDULE_COLOR_DUPLICATED":
+		"INVALID_SCHEDULE_PAYLOAD", "SCHEDULE_COLOR_DUPLICATED",
+		"INVALID_SETUP_SECRET", "SETUP_SECRET_NOT_CONFIGURED":
 		return http.StatusBadRequest
 	case "VERIFICATION_CODE_TOO_FREQUENT":
 		return http.StatusTooManyRequests