feat: replace user_gyms.role placeholder with temporal roles table (#140)

[ChristopherRotnes]

Summary

• Creates roles table with valid_from / valid_to for tracking instruktør tenure over time
• Migrates both existing user_gyms.role = 'instruktor' rows into roles; drops user_gyms.role column
• Adds fetchActiveRoles(buId) helper in db.js (returns rows where valid_from <= today AND (valid_to IS NULL OR valid_to >= today))
• Updates CLAUDE.md: removes role-placeholder caveat, adds roles architecture note

Test plan

• Verify user_gyms table no longer has a role column in Supabase dashboard
• Verify roles table has 2 rows (one per user), both with valid_to = null
• Verify sign-in still works (ensureGymMembership upserts correctly without the dropped column)
• Verify fetchActiveRoles() returns the correct active role

Closes #140

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