feat(deck-builder): /deck_code mint + /deck resolve with 3-min in-memory TTL

Adds the portal pair (shadowverse-portal.com deck-builder endpoints) as
anonymous routes on the app server. The translation middleware learns a new
[NoWireEncryption] attribute that skips both AES calls but keeps the rest of
the msgpack + base64 + envelope pipeline intact, matching prod's portal wire
profile observed in data_dumps/traffic_prod_deckcode.ndjson.

Storage is a 3-minute IMemoryCache — codes are anonymous-global, 4-char
lowercase alphanumeric (matches the shortest prod sample). Foil bit is
stripped on mint to match prod's normalize-on-encode behaviour.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Services/DeckCodeService.cs

@@ -0,0 +1,62 @@
+using Microsoft.Extensions.Caching.Memory;
+using SVSim.EmulatedEntrypoint.Models.Dtos.Responses.DeckBuilder;
+
+namespace SVSim.EmulatedEntrypoint.Services;
+
+/// <summary>
+/// In-memory deck-code store with a 3-minute absolute TTL. Codes are lowercase 4-character
+/// alphanumeric tokens — matches the shortest sample observed in prod (e.g. "t7rz" in
+/// data_dumps/traffic_prod_deckcode.ndjson). The portal's anonymous global namespace is
+/// mirrored here: codes are not scoped to viewer.
+/// </summary>
+public sealed class DeckCodeService : IDeckCodeService
+{
+    public static readonly TimeSpan Ttl = TimeSpan.FromMinutes(3);
+
+    private const string Alphabet = "abcdefghijklmnopqrstuvwxyz0123456789";
+    private const int CodeLength = 4;          // 36^4 ≈ 1.7M codes
+    private const int MaxMintAttempts = 8;     // collision retries — saturation is genuinely exceptional
+
+    private readonly IMemoryCache _cache;
+    private readonly IRandom _random;
+
+    public DeckCodeService(IMemoryCache cache, IRandom random)
+    {
+        _cache = cache;
+        _random = random;
+    }
+
+    public string Mint(DeckPayload payload)
+    {
+        for (int attempt = 0; attempt < MaxMintAttempts; attempt++)
+        {
+            string code = GenerateCode();
+            string key = CacheKey(code);
+            if (_cache.TryGetValue(key, out _)) continue;
+
+            _cache.Set(key, payload, Ttl);
+            return code;
+        }
+
+        // Hit only if the 4-char namespace is genuinely saturated within a 3-minute window.
+        // At that load we'd want longer codes; throw loudly so the symptom doesn't get buried.
+        throw new InvalidOperationException(
+            $"Deck-code namespace saturated after {MaxMintAttempts} attempts. " +
+            "Either traffic exploded or the cache is misconfigured.");
+    }
+
+    public DeckPayload? TryResolve(string code)
+        => _cache.TryGetValue<DeckPayload>(CacheKey(code), out var payload) ? payload : null;
+
+    private string GenerateCode()
+    {
+        Span<char> buf = stackalloc char[CodeLength];
+        for (int i = 0; i < CodeLength; i++)
+        {
+            buf[i] = Alphabet[_random.Next(Alphabet.Length)];
+        }
+        return new string(buf);
+    }
+
+    internal static string CacheKey(string code) => $"deck_code:{code}";
+}