Fix duplicate invites

[tonyvince]

Fix #1436

[Shpigford]

This pull request introduces a new validation to ensure that an email can only be invited once per family and adds a unique index to enforce this constraint at the database level. The most important changes include updating the Invitation model and modifying the database schema to include the new unique index.

Model validation updates:

• app/models/invitation.rb: Added a validation to ensure the uniqueness of the email within the scope of family_id.

Database schema updates:

• db/migrate/20241108150422_add_unique_email_index_to_invitations.rb: Created a migration to add a unique index on the email and family_id columns in the invitations table.
• db/schema.rb: Updated the schema to reflect the new unique index on the email and family_id columns in the invitations table. [1] [2]

[Shpigford]

Thanks @tonyvince!