feat: implement multisig proposal queue API (#961)

[zainabwahab-eth]

Closes #961

Summary

Implements the multi-sig proposal queue API for managing admin proposals requiring multi-sig approval.

Endpoints Implemented

• GET /admin/proposals - Paginated proposal queue with status filter (pending/executed/rejected)
• GET /admin/proposals/:id - Detailed proposal view with all signatures and signer addresses
• POST /admin/proposals/:id/sign - Submit signature/approval, triggers execution when threshold reached
• POST /admin/proposals/:id/reject - Reject proposal with rejector address and timestamp

Key Features

• Signer Authorization: Uses existing ADMIN_MULTISIG_WALLETS env config (2-of-3 default) for authorization
• Threshold Execution: Automatic execution when required distinct authorized signatures are collected
• Duplicate Prevention: DB-level unique constraint on (proposalId, signer) prevents duplicate signatures
• Concurrency Safety: Uses Prisma transactions for atomic signature counting and state transitions
• Proposal Lifecycle: Enforces valid state transitions (pending → executed/rejected), rejects invalid transitions
• Audit Trail: All actions logged via existing audit infrastructure

Database Changes

• New multisig_proposals table with proposal metadata, status, threshold, proposer
• New multisig_signatures table with foreign key to proposals, unique constraint on (proposalId, signer)
• Migration: 20260927000000_add_multisig_proposal_queue

Tests

• Comprehensive integration tests covering all endpoints and error cases
• Unit tests for service layer covering threshold logic, authorization, state transitions
• Concurrency test for race condition prevention around threshold execution

Documentation

• Updated docs/api-endpoints.md with all new endpoints, request/response examples, and error codes

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