Add trackable raid elements

[jedmund]

Summary

• Add trackable boolean to raids for endgame raid tracking
• Create user_raid_elements table for crew element coverage tracking
• Add UserRaidElement model with validations and uniqueness constraints
• Add controller with index, sync (bulk replace), and for_user endpoints
• Add blueprint with raid_id, element, and computed raid_name

Test plan

• Run rails db:migrate for both migrations
• Verify GET /user_raid_elements returns grouped elements
• Verify PUT /user_raid_elements/sync replaces elements for a raid
• Verify GET /users/:username/raid_elements works without auth
• Run bundle exec rspec spec/models/user_raid_element_spec.rb spec/requests/api/v1/user_raid_elements_spec.rb