fix(import): tolerate numeric my_rotation_id; skip empty deck slots

A real /load/index dump emits my_rotation_id as a bare number (0) for
unset MyRotation slots, which 400'd against the string? DTO field
(AllowReadingFromString only covers string->number). FlexibleStringConverter
accepts either form. Also skip empty deck slots (no cards) on import — a
dump carries every slot, mostly empty placeholders the client manages
itself.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Models/Dtos/Common/FlexibleStringConverter.cs

@@ -0,0 +1,29 @@
+using System.Globalization;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Common;
+
+/// <summary>
+/// Reads a JSON string OR number as a nullable string, tolerating prod's polymorphic id fields.
+/// <c>rotation_id</c> on a /load/index <c>UserDeck</c> is a numeric string ("10008") for real
+/// MyRotation decks but a bare number (<c>0</c>) for unset slots — and the global
+/// <c>AllowReadingFromString</c> only covers the string→number direction, not number→string, so a
+/// plain <c>string?</c> property 400s on the numeric form. Null stays null; numbers serialize via
+/// invariant culture so a captured <c>0</c> round-trips to <c>"0"</c>.
+/// </summary>
+public sealed class FlexibleStringConverter : JsonConverter<string?>
+{
+    public override string? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) =>
+        reader.TokenType switch
+        {
+            JsonTokenType.Null => null,
+            JsonTokenType.String => reader.GetString(),
+            JsonTokenType.Number when reader.TryGetInt64(out var n) => n.ToString(CultureInfo.InvariantCulture),
+            JsonTokenType.Number => reader.GetDouble().ToString(CultureInfo.InvariantCulture),
+            _ => throw new JsonException($"Unexpected token {reader.TokenType} for a string-or-number field.")
+        };
+
+    public override void Write(Utf8JsonWriter writer, string? value, JsonSerializerOptions options) =>
+        writer.WriteStringValue(value);
+}

SVSim.EmulatedEntrypoint/Models/Dtos/Requests/Admin/ImportViewerRequest.cs

@@ -1,4 +1,5 @@
 using System.Text.Json.Serialization;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Common;
 
 namespace SVSim.EmulatedEntrypoint.Models.Dtos.Requests.Admin;
 
@@ -44,7 +45,11 @@ public class ImportDeck
     [JsonPropertyName("sleeve_id")]             public long? SleeveId { get; set; }
     [JsonPropertyName("leader_skin_id")]        public int? LeaderSkinId { get; set; }
     [JsonPropertyName("is_random_leader_skin")] public int? IsRandomLeaderSkin { get; set; }
-    [JsonPropertyName("my_rotation_id")]        public string? MyRotationId { get; set; }
+    // Prod emits rotation_id as a numeric string ("10008") for real MyRotation decks but a bare
+    // number (0) for unset slots; FlexibleStringConverter accepts either (a plain string? 400s on
+    // the numeric form because AllowReadingFromString only covers string→number).
+    [JsonPropertyName("my_rotation_id")] [JsonConverter(typeof(FlexibleStringConverter))]
+    public string? MyRotationId { get; set; }
 }
 
 public class ImportCurrency