docs: add multisig architecture guide

[JeremyZeng77]

Summary

• add docs/architecture/multisig.md covering the multisig contract, access-control variant, backend API, MongoDB proposal model, and client service boundary
• include Mermaid flow and sequence diagrams for proposal/approval/execution and upgrade queueing through Timelock
• document security invariants, operational drift risks, route naming notes, and verification pointers

Verification

• Checked Markdown code fences are balanced
• Confirmed the new file is ASCII-only and ends with a newline
• Reviewed references against contracts/multisig/src/lib.rs, contracts/access/src/multisig.rs, server/routes/multisig-routes.js, server/services/multisig-service.js, server/models/MultiSigTransaction.js, and client/src/services/multisigService.js

Closes #714