Add multisig proposal REST API endpoints

[codebestia]

Create apps/backend/src/routes/treasury.ts with authenticated endpoints: POST /treasury/propose, POST /treasury/proposals/:id/approve, POST /treasury/proposals/:id/reject, POST /treasury/proposals/:id/execute, GET /treasury/proposals?conversationId=, GET /treasury/proposals/:id. Each mutating endpoint calls the corresponding on-chain contract function via Stellar SDK and syncs the DB. Mount under /treasury.

Acceptance criteria:

• All routes return 403 for non-members
• POST /treasury/propose returns { id, onChainId, status: 'pending' }
• DB row status stays in sync with on-chain status

[drips-wave]

Congratulations, @h53566186-web! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @h53566186-web: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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