Support inviting users with custom attributes from UI and CLI

[nishantmonu51]

Custom attributes previously could only be set with rill user set-attributes after the invited user signed up. This carries attributes through the invite lifecycle so they can be set at invite time and are applied automatically on acceptance.

• Adds an attributes JSONB column to org_invites (migration 0096) with the same validation as member attributes; no change to project_invites since attributes are org-membership scoped.
• AddOrganizationMemberUser and AddProjectMemberUser accept an optional attributes field: stored on the (guest) org invite for new users, applied directly for existing users. Re-inviting updates attributes only when explicitly provided. Setting attributes via the project path requires ManageOrgMembers, matching the permission for UpdateOrganizationMemberUserAttributes.
• UpdateOrganizationMemberUserAttributes (and rill user set-attributes) now falls back to updating a pending invite, mirroring the existing SetOrganizationMemberUserRole behavior.
• rill user add gains --attribute key=value and --json flags for org and project adds.
• Web-admin: the org "Add users" and "Add guests" dialogs get an optional collapsed custom-attributes editor, and the org users table shows an attributes chip (with tooltip) for members and pending invites. OrganizationInvite exposes attributes for this.

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!