chore(med): mapping culture-safe parse, dead-code, scope comparer, UA rotator, parser cache

Six MEDIUM-tier review items:

* Mapping.cs — DateTimeOffset.Parse now passes CultureInfo.InvariantCulture
  + DateTimeStyles.RoundtripKind so a non-en-US thread culture cannot
  corrupt round-tripped ScheduledAt / CapturedAt / DetectedAt / CompletedAt.
  Also replaces the magic 0/1 BetScope discriminator with named constants.

* Delete dead Placeholder.cs files in Marathon.Application and
  Marathon.Infrastructure — they were stubs from Phase 1 to satisfy
  "non-empty project" and have been dead since Phase 2/3.

* EventBrowsingService — drop the bespoke ScopeEqualityComparer; BetScope
  is a record hierarchy, .GroupBy uses value equality natively.

* UserAgentRotatorHandler — counter promoted to private static int with
  Interlocked.Increment so rotation is round-robin across the process.
  HttpClientFactory builds the handler Transient, so the previous instance
  field reset to zero on every new client and broke rotation.

* EventOddsParser — added a parallel "selection-key → IElement" index
  alongside the existing price index. Handicap extraction (6 call sites
  per event detail page) used to do a fresh document.QuerySelector("span[
  data-selection-key='...']") for every key — full-document CSS traversal.
  Now it's a dictionary lookup, with the pair-emit logic factored into a
  shared TryEmitHandicapPair helper.

src/Marathon.Infrastructure/Persistence/Mapping.cs

@@ -1,3 +1,4 @@
+using System.Globalization;
 using Marathon.Domain.Entities;
 using Marathon.Domain.Enums;
 using Marathon.Domain.ValueObjects;
@@ -11,6 +12,15 @@ namespace Marathon.Infrastructure.Persistence;
 /// </summary>
 internal static class Mapping
 {
+    // ScheduledAt / CapturedAt / DetectedAt / CompletedAt are written via
+    // DateTimeOffset.ToString("O") — round-trip ISO 8601. Parse with the
+    // invariant culture and RoundtripKind so a non-en-US thread culture
+    // (or a future locale change) cannot corrupt the round-trip.
+    private const DateTimeStyles RoundtripStyles = DateTimeStyles.RoundtripKind;
+
+    // ─── Bet scope discriminator constants ────────────────────────────────────
+    private const int ScopeMatch  = 0;
+    private const int ScopePeriod = 1;
     // ─── Event ───────────────────────────────────────────────────────────────
 
     public static EventEntity ToEntity(Event domain) =>
@@ -34,7 +44,7 @@ internal static class Mapping
             CountryCode: entity.CountryCode,
             LeagueId: entity.LeagueId,
             Category: entity.Category,
-            ScheduledAt: DateTimeOffset.Parse(entity.ScheduledAt),
+            ScheduledAt: DateTimeOffset.Parse(entity.ScheduledAt, CultureInfo.InvariantCulture, RoundtripStyles),
             Side1Name: entity.Side1Name,
             Side2Name: entity.Side2Name)
         {
@@ -55,7 +65,7 @@ internal static class Mapping
     public static OddsSnapshot ToDomain(SnapshotEntity entity) =>
         new(
             eventId: new EventId(entity.EventCode),
-            capturedAt: DateTimeOffset.Parse(entity.CapturedAt),
+            capturedAt: DateTimeOffset.Parse(entity.CapturedAt, CultureInfo.InvariantCulture, RoundtripStyles),
             source: (OddsSource)entity.Source,
             bets: entity.Bets.Select(ToDomain).ToList().AsReadOnly());
 
@@ -64,7 +74,7 @@ internal static class Mapping
     public static BetEntity ToEntity(Bet domain) =>
         new()
         {
-            Scope = domain.Scope is MatchScope ? 0 : 1,
+            Scope = domain.Scope is MatchScope ? ScopeMatch : ScopePeriod,
             PeriodNumber = domain.Scope is PeriodScope ps ? ps.Number : null,
             Type = (int)domain.Type,
             Side = (int)domain.Side,
@@ -76,8 +86,8 @@ internal static class Mapping
     {
         var scope = entity.Scope switch
         {
-            0 => (BetScope)MatchScope.Instance,
-            1 => new PeriodScope(entity.PeriodNumber!.Value),
+            ScopeMatch  => (BetScope)MatchScope.Instance,
+            ScopePeriod => new PeriodScope(entity.PeriodNumber!.Value),
             _ => throw new InvalidOperationException(
                 $"Unknown BetScope discriminator: {entity.Scope}"),
         };
@@ -108,7 +118,7 @@ internal static class Mapping
             Side1Score: entity.Side1Score,
             Side2Score: entity.Side2Score,
             WinnerSide: (Side)entity.WinnerSide,
-            CompletedAt: DateTimeOffset.Parse(entity.CompletedAt));
+            CompletedAt: DateTimeOffset.Parse(entity.CompletedAt, CultureInfo.InvariantCulture, RoundtripStyles));
 
     // ─── Anomaly ──────────────────────────────────────────────────────────────
 
@@ -127,7 +137,7 @@ internal static class Mapping
         new(
             Id: Guid.Parse(entity.Id),
             EventId: new EventId(entity.EventCode),
-            DetectedAt: DateTimeOffset.Parse(entity.DetectedAt),
+            DetectedAt: DateTimeOffset.Parse(entity.DetectedAt, CultureInfo.InvariantCulture, RoundtripStyles),
             Kind: (AnomalyKind)entity.Kind,
             Score: entity.Score,
             EvidenceJson: entity.EvidenceJson);