test+chore: real-SQLite query coverage, batch detect writes, finish date centralization

Review follow-ups:
- (HIGH) Add real-SQLite round-trip tests for the new query methods so the load-bearing
  lexical O-format date ordering is verified, not just mocked: Anomaly
  ListByDateRange/CountSince, Snapshot CountSince/ListByEvents grouping, Event Query/GetMany.
- (MED) DetectAnomaliesUseCase: one SaveChanges per event instead of per anomaly.
- (LOW) Route PlacedBetRepository + ExcelExporter date bounds through SqliteDateText.
- (LOW) Backtest: reject a one-sided date range (was silently ignored).
- (LOW) Refresh stale comments after the detector fan-out.

src/Marathon.Application/UseCases/DetectAnomaliesUseCase.cs

@@ -14,7 +14,7 @@ namespace Marathon.Application.UseCases;
 ///   <item>Loads all tracked events.</item>
 ///   <item>For each event, fetches its last-24-hour live snapshots.</item>
 ///   <item>Runs <see cref="AnomalyDetector"/> over the snapshot timeline.</item>
-///   <item>Persists any new anomalies that have not already been stored (dedup by EventId + DetectedAt minute-window).</item>
+///   <item>Persists any new anomalies that have not already been stored (dedup by EventId + Kind + DetectedAt minute-window).</item>
 /// </list>
 /// </summary>
 /// <remarks>
@@ -138,8 +138,8 @@ public sealed class DetectAnomaliesUseCase
         List<Anomaly> existingForEvent,
         CancellationToken ct)
     {
-        // Fan out over every detector kind; dedup below keys on EventId + Kind so the
-        // flip and steam signals for one event persist independently.
+        // Fan out over every detector; dedup below keys on EventId + Kind so the flip,
+        // steam, and freeze signals for one event persist independently.
         var detected = detectors
             .SelectMany(d => d.Detect(ev.Id, snapshots))
             .ToList();
@@ -154,11 +154,15 @@ public sealed class DetectAnomaliesUseCase
                 continue;
 
             await _anomalyRepo.AddAsync(anomaly, ct);
-            await _anomalyRepo.SaveChangesAsync(ct);
             existingForEvent.Add(anomaly); // Keep local list in sync so the same cycle doesn't re-add.
             persisted++;
         }
 
+        // One write per event rather than per anomaly — with three detectors an event
+        // can yield several new anomalies in a single cycle.
+        if (persisted > 0)
+            await _anomalyRepo.SaveChangesAsync(ct);
+
         return persisted;
     }