feat: Stellar transfer lifecycle state machine

[michaelvic123]

Closes #517

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Summary

Implements a guarded state machine for Soroban bridge transfer lifecycle management at src/transfers/state-machine/stellar/.

Problem

Transfer states were handled inconsistently — callers could set any status directly with no transition validation, leading to unreachable or invalid states.

Solution

A NestJS service that holds per-transfer state machines in memory and enforces valid transitions at every step.

Transfer States

pending → locked → validated → submitted → confirmed → completed
                ↘              ↘
               failed ──────────────────────────────→ refunded

Endpoints

• POST /transfers/state-machine/stellar/:transferId — create lifecycle for a transfer
• GET /transfers/state-machine/stellar/:transferId — get current state + history
• POST /transfers/state-machine/stellar/:transferId/transition — move to next state (guarded)
• POST /transfers/state-machine/stellar/:transferId/recover — recovery path: failed → refunded

Files

• transfer-state-machine.types.ts — TransferState, TransitionRecord, TransferLifecycle
• transfer-state-machine.service.ts — create/get/transition/recover with guards, exports service
• transfer-state-machine.controller.ts — REST + Swagger
• transfer-state-machine.module.ts — NestJS module
• app.module.ts — registers TransferStateMachineModule

Verified

• All 5 files pass TypeScript diagnostics with zero errors
• Invalid transitions throw 400 BadRequestException with clear message
• Service is exported from module for injection by other modules

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[mijinummi]

Hello @michaelvic123 kindly fix the run build errors