Implement multi-sig admin governance

[Prasiejames]

Problem

All 4 contracts have a single-admin model (require_admin()). If the admin key is compromised or lost, the entire protocol is at risk. There is no recovery path.

Expected Behavior

Admin operations should require M-of-N multi-signature approval for sensitive operations (upgrade, pause, transfer limits, dividend distribution).

Proposed Solution

Replace the single Address admin with a SorobanMultiSig or a custom multi-sig contract pattern:

1. Add an AdminManager contract (or embed the logic) that tracks signers and threshold
2. Propose operations via submit_proposal(operation_hash)
3. Execute after M signers approve via approve(proposal_id)
4. The existing set_admin could become set_admin_manager

This could use the Soroban multi-sig pattern or a custom implementation specific to this protocol.

Affected Files

• All 4 contracts (require_admin pattern)
• Potentially a new admin-manager contract