fix(maintenance-pool): guard assignReward by issueId and enforce unique payout index (#458)

[Proxima84-code]

Resolves #458 (Regression on #273).

Root Cause

In MaintenancePoolService.assignReward(), the idempotency guard existingPoolPayment filtered on payment.recipientId = :recipientId instead of payment.maintenanceIssueId = :issueId. This caused:

1. The same maintenance issue to be rewarded multiple times when assigned to different recipients.
2. Legitimate payouts for multiple distinct issues to the same recipient to be rejected with ConflictException.
3. Anonymous payouts (recipientId = null) to bypass the guard entirely.
4. An unused dead query (existingPayment) executing on every call.

Fix

• Entity: Added nullable maintenanceIssueId and maintenanceIssue relation to Payment, along with composite unique index UQ_payment_escrow_maintenance_issue on [escrowId, maintenanceIssueId] to close concurrent race conditions at the database level.
• EscrowService: Added optional maintenanceIssueId parameter to poolWithdraw() and persisted it on the Payment row. Fixed Soroban u64 deadline expectation in test fixtures.
• MaintenancePoolService:
  • Removed dead existingPayment query.
  • Updated existingPoolPayment to filter by payment.maintenanceIssueId = :issueId.
  • Passed issueId to poolWithdraw().
  • Handled PostgreSQL unique constraint violations (23505) on concurrent races, converting them into ConflictException and restoring pool balance via compensating increment (assignReward decrements MaintenancePool.balance before poolWithdraw() and never restores it when the withdrawal fails #459).
• Test Suite:
  • Fixed missing ACTIVE status on pool fixtures in maintenance-pool.service.spec.ts.
  • Updated test MaintenancePool.balance is a hand-maintained running total with a lost-update race across concurrent deposit/assignReward calls #51 to test concurrency across distinct issues.
  • Added test coverage for: duplicate issue with different recipient (rejected), multiple issues for the same recipient (allowed), anonymous payouts (guarded), and unique constraint race recovery.
  • Verified: 20/20 in maintenance-pool.service.spec.ts and 58/58 in escrow.service.spec.ts.

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[Proxima84-code]

Hi @maintainers,

Since the deadline for issue #458 passed on September 30 without a solution from the original assignee, I have submitted a complete and fully tested fix here (all 78 unit tests passing locally, production build verified).

Could you please approve the CI workflow run and review the PR? Thank you!