perf(ui): batch event-list snapshot loads + push distinct dimensions to DB (HIGH)

* EventBrowsingService.BuildListAsync issued one snapshot query per event
  on every page render — N+1 against SQLite, with each round-trip hauling
  the full bet graph via Include(Bets). Replaced with a single
  ISnapshotRepository.ListByEventsAsync batch.
* ListKnownSportCodesAsync / ListKnownCountryCodesAsync used to materialise
  every Event row to compute Distinct() in memory. Pushed to EF projection
  via two new IEventRepository methods (ListDistinctSportCodesAsync,
  ListDistinctCountryCodesAsync) implemented as
  .Select(...).Distinct().ToListAsync — single SELECT DISTINCT.

src/Marathon.Application/Abstractions/IEventRepository.cs

@@ -12,4 +12,16 @@ public interface IEventRepository : IRepository<EventId, Event>
     Task<IReadOnlyList<Event>> ListByDateRangeAsync(DateRange range, CancellationToken ct = default);
 
     Task<IReadOnlyList<Event>> ListBySportAsync(SportCode sport, CancellationToken ct = default);
+
+    /// <summary>
+    /// Distinct sport codes across the events table. Projects in the database
+    /// rather than materialising every <see cref="Event"/> on the client.
+    /// </summary>
+    Task<IReadOnlyList<int>> ListDistinctSportCodesAsync(CancellationToken ct = default);
+
+    /// <summary>
+    /// Distinct ISO-2 country codes across the events table. Projects in the
+    /// database rather than materialising every <see cref="Event"/>.
+    /// </summary>
+    Task<IReadOnlyList<string>> ListDistinctCountryCodesAsync(CancellationToken ct = default);
 }

src/Marathon.Infrastructure/Persistence/Repositories/EventRepository.cs

@@ -50,6 +50,28 @@ internal sealed class EventRepository : IEventRepository
         return entities.Select(Mapping.ToDomain).ToList().AsReadOnly();
     }
 
+    public async Task<IReadOnlyList<int>> ListDistinctSportCodesAsync(CancellationToken ct = default)
+    {
+        var codes = await _db.Events.AsNoTracking()
+            .Select(e => e.SportCode)
+            .Distinct()
+            .ToListAsync(ct);
+
+        codes.Sort();
+        return codes;
+    }
+
+    public async Task<IReadOnlyList<string>> ListDistinctCountryCodesAsync(CancellationToken ct = default)
+    {
+        var codes = await _db.Events.AsNoTracking()
+            .Select(e => e.CountryCode)
+            .Distinct()
+            .ToListAsync(ct);
+
+        codes.Sort(StringComparer.OrdinalIgnoreCase);
+        return codes;
+    }
+
     public async Task AddAsync(Event entity, CancellationToken ct = default)
     {
         var efEntity = Mapping.ToEntity(entity);

src/Marathon.UI/Services/EventBrowsingService.cs

@@ -68,25 +68,11 @@ public sealed class EventBrowsingService : IEventBrowsingService
             history);
     }
 
-    public async Task<IReadOnlyList<int>> ListKnownSportCodesAsync(CancellationToken ct)
-    {
-        var all = await _events.ListAsync(ct).ConfigureAwait(false);
-        return all
-            .Select(static e => e.Sport.Value)
-            .Distinct()
-            .OrderBy(static x => x)
-            .ToList();
-    }
+    public Task<IReadOnlyList<int>> ListKnownSportCodesAsync(CancellationToken ct)
+        => _events.ListDistinctSportCodesAsync(ct);
 
-    public async Task<IReadOnlyList<string>> ListKnownCountryCodesAsync(CancellationToken ct)
-    {
-        var all = await _events.ListAsync(ct).ConfigureAwait(false);
-        return all
-            .Select(static e => e.CountryCode)
-            .Distinct()
-            .OrderBy(static x => x, StringComparer.OrdinalIgnoreCase)
-            .ToList();
-    }
+    public Task<IReadOnlyList<string>> ListKnownCountryCodesAsync(CancellationToken ct)
+        => _events.ListDistinctCountryCodesAsync(ct);
 
     // ---------------- internals ----------------
 
@@ -121,18 +107,26 @@ public sealed class EventBrowsingService : IEventBrowsingService
 
         var sorted = ApplySort(filtered, filter.SortKey, filter.SortDescending);
         var materialized = sorted.ToList();
+        if (materialized.Count == 0)
+            return Array.Empty<EventListItem>();
 
-        // Read each event's latest matching snapshot to populate the preview odds.
+        // Single batched snapshot query for all events on the page (replaces the
+        // prior per-event ListByEventAsync round-trip — N+1 against SQLite). The
+        // repository fills empty entries for events with no snapshots in range.
         var rangeFrom = filter.Dates.From.AddDays(-2);
         var rangeTo = filter.Dates.To.AddDays(2);
+        var ids = materialized.Select(e => e.Id).ToList();
+        var snapshotsByEvent = await _snapshots
+            .ListByEventsAsync(ids, rangeFrom, rangeTo, ct)
+            .ConfigureAwait(false);
 
         var rows = new List<EventListItem>(materialized.Count);
         foreach (var ev in materialized)
         {
             ct.ThrowIfCancellationRequested();
-            var snapshots = await _snapshots
-                .ListByEventAsync(ev.Id, rangeFrom, rangeTo, ct)
-                .ConfigureAwait(false);
+            var snapshots = snapshotsByEvent.TryGetValue(ev.Id, out var found)
+                ? found
+                : Array.Empty<OddsSnapshot>();
 
             var matching = snapshots
                 .Where(s => s.Source == source)