Singleton pattern for flush metrics queue handlers

[tudddorrr]

No description provided.

[claude]

🔴 Critical

Concurrent EntityManager usage creates race condition

Location: src/lib/queues/game-metrics/flush-metrics-queue-handler.ts:27-28

The change removes em.fork() when calling checkGroupMemberships in parallel:

// Before (sequential with isolated EMs)
for (const player of players) {
  await player.checkGroupMemberships(em.fork())
}

// After (parallel with shared EM)
const promises = players.map((player) => player.checkGroupMemberships(em))
await Promise.all(promises)

Problem: checkGroupMemberships modifies the EntityManager's UnitOfWork by:

• Calling player.groups.add() / player.groups.remove() (checkGroupMemberships.ts:41-43)
• Calling em.flush() internally (checkGroupMemberships.ts:80)

Sharing the same em across parallel operations can cause race conditions where multiple concurrent flushes and entity modifications interfere with each other.

Fix: Keep the parallelization for performance, but restore em.fork() for isolation:

const promises = players.map((player) => player.checkGroupMemberships(em.fork()))
await Promise.all(promises)

---

🟡 Major

No issues found

---

🔵 Minor

No issues found

---

Overall Assessment

The singleton pattern for queue handlers is a reasonable optimization since services are instantiated once. However, the concurrent EntityManager change introduces a critical bug. After fixing the EM issue, this PR will be good to merge.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.21%. Comparing base (330f5bc) to head (a8376db).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #720   +/-   ##
========================================
  Coverage    99.21%   99.21%           
========================================
  Files          225      225           
  Lines        16602    16607    +5     
  Branches      1536     1535    -1     
========================================
+ Hits         16471    16476    +5     
  Misses         128      128           
  Partials         3        3           

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