fix(maintenance-pool): drop broken pessimistic lock in deposit (#457)

[Proxima84-code]

Resolves #457.

Root Cause

In MaintenancePoolService.deposit(), createQueryBuilder('pool').setLock('pessimistic_write') was executed outside an active transaction. TypeORM throws PessimisticLockTransactionRequiredError when executing pessimistic locks without an active transaction runner, causing all POST /maintenance-pool/:id/deposit requests to crash with HTTP 500. Furthermore, wrapping the entire call in a database transaction would hold database row locks during multi-second Soroban RPC network calls (escrowService.fund).

Fix

• Dropped the broken pessimistic lock in deposit() and replaced it with a direct findOne() read.
• Leveraged the existing atomic conditional Compare-And-Set (update({ id, escrowId: null }, { escrowId })), which already handles concurrent first-time deposit races safely without connection starvation.
• Restored valid syntax in escrow.service.ts and updated maintenance-pool.service.spec.ts.

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