Support adding users to user groups at invitation time (AddOrganizationMemberUser / AddProjectMemberUser usergroups)

[nishantmonu51]

Admins can now assign user groups when inviting a user, instead of inviting, waiting for acceptance, and then adding them to groups. Follows the pattern used for invite-time attributes in #9797. The org_invites.usergroup_ids column and its application on signup already existed; this PR exposes it through the APIs, CLI, and UI.

Backend

• AddOrganizationMemberUser and AddProjectMemberUser accept usergroups (group names). For a pending signup they are stored on the org invite and merged additively on re-invite; for an existing user they are applied immediately in the same transaction (ON CONFLICT DO NOTHING, so it is safe inside the tx).
• A shared resolveUsergroupsForMembership validates groups (exist, not managed, admin-role guard) and is reused by AddUsergroupMemberUser, so the rules cannot drift.
• OrganizationInvite exposes usergroups; ListUsergroupMemberUsers includes pending invitees with pending_acceptance; RemoveUsergroupMemberUser handles pending invitees; DeleteUsergroup scrubs the group from pending invites.
• No migration needed. Tests added in rbac_test.go and postgres_test.go.

CLI

• rill user add --group is repeatable and is sent with the org or project invite in a single call. The interactive fallback no longer drops --attribute values.
• rill user list shows groups and attributes on pending invites, and a status column for group members.

Frontend

• UserGroupsMultiSelect in the Add users and Add guests dialogs.
• Pending rows show their invite groups; a new ManageUserGroupsDialog is reachable from a "Manage groups" action on every row of the users and guests tables.
• The Create and Edit group dialogs list pending invitees and can add them via search.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!