Add substitutions feature

[jedmund]

Summary

• Adds roles table (admin-managed labels for grid slots) and substitutions join table
• Grid items gain is_substitute, role_id, and substitution_note fields
• Scoped party associations exclude substitutes from counts/queries
• Substitution remapping on remix via amoeba
• CRUD endpoints for substitutions, read-only roles endpoint
• Blueprints render role, note, and substitutions in nested views

Test plan

• rails db:migrate succeeds
• 16 new specs pass (role/substitution models + request specs)
• Manual test: create, reorder, and delete substitutions
• Manual test: remix party with substitutions