feat(ops): pipeline-health dashboard

- Add a /health ops page: snapshot freshness (last-capture-at, colour-coded), 24h vs
  total snapshots + anomalies, events tracked, sports covered, and the four worker
  on/off states. Nav entry under System; localized en+ru.
- New IPipelineHealthService + ISnapshotRepository.GetLatestCapturedAtAsync (max
  CapturedAt via indexed ORDER BY/LIMIT 1), with a real-SQLite round-trip test.

tests/Marathon.Infrastructure.Tests/Persistence/RoundTripTests.cs

@@ -366,6 +366,29 @@ public sealed class RoundTripTests : IDisposable
         many.Keys.Select(k => k.Value).Should().BeEquivalentTo(new[] { "Q1", "Q3" });
     }
 
+    [Fact]
+    public async Task Snapshot_GetLatestCapturedAt_ReturnsMostRecentOrNull()
+    {
+        // Empty store → null.
+        (await _snapshotRepo.GetLatestCapturedAtAsync()).Should().BeNull();
+
+        await _eventRepo.AddAsync(BuildEvent("L100"));
+        await _eventRepo.SaveChangesAsync();
+
+        var older = new DateTimeOffset(2026, 5, 1, 12, 0, 0, MoscowOffset);
+        var newer = new DateTimeOffset(2026, 5, 9, 18, 30, 0, MoscowOffset);
+        OddsSnapshot Snap(DateTimeOffset at) =>
+            new(new EventId("L100"), at, OddsSource.Live,
+                new List<Bet> { new(MatchScope.Instance, BetType.Win, Side.Side1, null, new OddsRate(1.90m)) });
+
+        await _snapshotRepo.AddAsync(Snap(older));
+        await _snapshotRepo.AddAsync(Snap(newer));
+        await _snapshotRepo.SaveChangesAsync();
+        _fixture.DbContext.ChangeTracker.Clear();
+
+        (await _snapshotRepo.GetLatestCapturedAtAsync()).Should().Be(newer);
+    }
+
     // ── Helpers ─────────────────────────────────────────────────────────────
 
     private static Event BuildEvent(string id, DateTimeOffset? scheduledAt = null) =>