feat: add AddOrganizationMembers RPC for batch member addition with explicit roles

AddOrganizationUsers lacks a role parameter and hardcodes the default
viewer role. AddOrganizationMembers accepts a list of {user_id, role_id}
pairs so callers can specify the role at add time. Returns per-member
success/error results for partial failure handling.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: whoAbhishekSah

raystack/frontier/v1beta1/frontier.proto

@@ -133,6 +133,8 @@ service FrontierService {
 
   rpc SetOrganizationMemberRole(SetOrganizationMemberRoleRequest) returns (SetOrganizationMemberRoleResponse) {}
 
+  rpc AddOrganizationMembers(AddOrganizationMembersRequest) returns (AddOrganizationMembersResponse) {}
+
   rpc GetOrganizationKyc(GetOrganizationKycRequest) returns (GetOrganizationKycResponse) {}
 
   // Deprecated: use ListServiceUsers instead
@@ -1597,6 +1599,26 @@ message SetOrganizationMemberRoleRequest {
 
 message SetOrganizationMemberRoleResponse {}
 
+message AddOrganizationMembersRequest {
+  string org_id = 1 [(buf.validate.field).string.uuid = true];
+  repeated OrgMemberEntry members = 2;
+}
+
+message OrgMemberEntry {
+  string user_id = 1 [(buf.validate.field).string.uuid = true];
+  string role_id = 2 [(buf.validate.field).string.uuid = true];
+}
+
+message AddOrganizationMembersResponse {
+  repeated OrgMemberResult results = 1;
+}
+
+message OrgMemberResult {
+  string user_id = 1;
+  bool success = 2;
+  string error = 3;
+}
+
 message ListOrganizationServiceUsersRequest {
   string id = 1 [(buf.validate.field).string.min_len = 3];
 }