WrzDJSet: Collaboration — invite-link editors via SetCollaborator

[thewrz]

Context

SetCollaborator is modeled since Phase 0 but unenforced; v3 turns on real multi-DJ collaboration. Exec summary §2 (Collaboration decision), §8.

Scope

• Invite-link flow: owner generates editor/viewer invite, recipient accepts with their DJ account
• Role enforcement on every /api/setbuilder/* mutation (editor) vs read (viewer)
• Presence/conflict strategy for concurrent edits (last-write-wins + activity feed at minimum)
• Collaborator management UI (list, change role, revoke)
• sharing_mode "invite_only" enforced end-to-end

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Work Estimate

• Estimated effort: 48 hours (~6 workdays).
• Planning phase: v3 Scale.
• Planning milestone: v3-Scale.
• Workplan target: 2026-09-10 milestone due date.
• Basis: Covers invite flow, editor/viewer authorization, collaboration management UI, and a minimal concurrent-edit conflict strategy.