Make refresh ticket deduction atomic with opponent refresh

[Atralupus]

Problem

POST /available-opponents/refresh committed three steps separately:

1. RefreshAvailableOpponents (own transaction)
2. UpdateRefreshTicketStatusPerRound (read-modify-write on the tracked entity)
3. AddRefreshTicketUsageLog

This caused two issues observed on heimdall in season 44, round 1:

• Free refresh on partial failure: step 1 committed but step 2 failed, so a new opponent set was created without deducting a ticket (one set with no usage log, used_count one lower than the number of sets). This gave the user one extra +22 opportunity in the round.
• Lost deduction under concurrent requests: two overlapping refreshes both read the same remaining_count and wrote the same value back (5 usage logs vs used_count = 4). Because neither saw the other's uncommitted set, two opponent sets stayed active at the same time (10 active opponents).

Both affected records have been corrected manually in the DB.

Fix

• Add ITicketRepository.TryDeductRefreshTicket, which does a conditional ExecuteUpdate (remaining_count > 0).
• In the controller, run the deduction, opponent refresh and usage log in a single ReadCommitted transaction, deducting first. The UPDATE holds the status row lock until commit, so concurrent refreshes for the same avatar are serialized and the later one soft-deletes the set committed by the earlier one. If no ticket is left, it returns NO_REFRESH_TICKETS as before.
• Remove the inner transaction from RefreshAvailableOpponents. It is a single SaveChanges and now runs in the caller's transaction (a nested BeginTransaction would throw).

Test

• dotnet build passes
• dotnet test ArenaService.Tests passes (39/39)

🤖 Generated with Claude Code