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feat: add AddOrganizationMembers RPC handler with superadmin auth

Wire the membership package into the AddOrganizationMembers AdminService
RPC. Batch endpoint accepts list of {user_id, role_id} pairs and returns
per-member success/error results.

- Handler iterates members, calls membershipService.AddOrganizationMember
- Domain errors (already member, invalid role, etc.) returned as-is
- Internal errors masked with generic message and logged server-side
- Authorization: IsSuperUser (AdminService)
- Proto regenerated from proton branch with new RPC

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>