fix(phase-7): close review notes — hoist anomaly dedup query, drop dead expr

Phase 7 reviewer (Sonnet, combined backend + frontend) flagged 3 🟡 warnings;
two real fixes here, one tracking:

W1 — DetectAnomaliesUseCase had an undocumented N+1: _anomalyRepo.ListAsync
  was called inside ProcessEventAsync, once per event. Hoisted to ExecuteAsync
  before the loop and threaded into ProcessEventAsync as a parameter. The
  per-event slice happens in-memory now. O(N_events) DB round-trips → 1.

W2 — AnomalyDetector.ExtractMatchWinProbabilities had a dead expression
  '(decimal?)null ?? 0m' that always evaluated to 0m. Simplified to
  'drawBet is not null ? rawDraw / total : 0m'. The 0m is never surfaced
  anyway (PDraw in the return uses the same null guard), so behaviour is
  identical.

W3 — PLAN.md row updated with both Phase 7 commit hashes (a6ff368 backend
  + 12208a4 frontend) and review verdict.

Build 0/0, 276 tests still passing.

plans/initial-implementation/PLAN.md

@@ -69,7 +69,7 @@ parameter configurable.
 | Phase 4: Application + Workers | backend | ✅ Done | ⚠️ Pass with notes (Sonnet) | ✅ Build OK + 202/202 tests | ✅ 2acbaa5 |
 | Phase 5: Host + Theme + i18n | frontend | ✅ Done | ⚠️ Pass with notes (Sonnet, combined batch) | ✅ Build OK + 11/11 UI tests | ✅ batch (e4d8476…686550d…+) |
 | Phase 6: Event browsing UI | frontend | ✅ Done | ⚠️ Pass with notes (Sonnet) | ✅ Build OK + 228/228 tests | ✅ 553db2b |
-| Phase 7: Anomaly detection | fullstack | ✅ Done | ⬜ | ✅ Build OK + 276/276 tests | ⬜ |
+| Phase 7: Anomaly detection | fullstack | ✅ Done | ⚠️ Pass with notes (Sonnet) | ✅ Build OK + 276/276 tests | ✅ a6ff368 + 12208a4 |
 | Phase 8: Results loader | fullstack | ⬜ Not Started | ⬜ | ⏭️ Big Bang | ⬜ |
 | Phase 9: Packaging + polish | fullstack | ⬜ Not Started | ⬜ | ⬜ | ⬜ |