Collaborations: rework mobile feature on the unified Collaborator model (collaborations flag)

[teetangh]

Context

The web schema merged WebinarCollaborator/ClassCollaborator into a single Collaborator model (collaboratorType WEBINAR|CLASS, webinarPlanId/classPlanId XOR, typed permission booleans replacing the permissions JSON, revenueShareBps replacing revenueSharePercentage). backend/lib/database/repositories/collaborator_repository.dart was minimally rewritten to compile against the new model (read + respond only), and the feature is gated behind FeatureFlags.collaborations (false) with 403 middleware on collaborations/.

Scope

1. Full repository rework: expose typed permissions (canApprovePayment, canViewAnalytics, canEditEvent, canSeeAttendees) and bps-based revenue share end to end
2. Frontend entities/screens (lib/features/collaborations/, lib/domain/entities/collaborator/) currently expect revenueSharePercentage — the repo converts bps→percentage at the boundary as a stopgap; decide the long-term contract
3. Host-side management (invite, role change, removal) — see ChatGPT 5.4: Collaborator lifecycle review: invitee-only mobile contract, missing host management, and no post-accept provisioning #58 for the lifecycle gaps
4. Flip FeatureFlags.collaborations

Related: #58