feat(spot-card-exchange): /spot_card_exchange/{top,exchange} + SpotPoints currency

Final shop family. Schema additions:
- ViewerCurrency.SpotPoints (ulong) — new currency column on Viewers.
- SpotCardExchangeEntry — catalog (distinct from the pre-existing
  SpotCardEntry, which is the /load/index rental-cost concept).
- ViewerSpotCardExchange — standalone composite-PK table tracking
  (viewer, card, exchanged_at, is_pre_release_snapshot). Standalone
  avoids cartesian-explode on viewer-graph reads.

RewardGrantService gains a SpotCardPoint=12 currency case mirroring
the RedEther/Crystal pattern. Doc comment refreshed; SpotCard=11 and
SpotCardOnlyLatestCardPack=13 remain unimplemented with explanatory
NotSupportedException — captures show emitters always use Card=5 with
the spot-card-specific id.

Controller:
- /top: emits exactly 9 clan buckets [{"1": [cards]}, ...] matching
  prod's arbitrary single-key shape. exchange_status per-card (0=
  available, 1=already-exchanged, 2=LimitOver after pre-release cap).
  pre_relase_info WIRE TYPO PRESERVED ("relase" not "release").
- /exchange: server-authoritative price (client-supplied
  exchange_point ignored); debits SpotPoints with post-state-total
  reward_list entry; grants card via RewardGrantService.ApplyAsync
  (cosmetic cascade included); persists ViewerSpotCardExchange row.
  Insufficient points / already-exchanged / pre-release-limit all
  return 400 without partial state.

LoadController now populates /load/index spot_point from
viewer.Currency.SpotPoints (was always 0).

PreReleaseLimit hardcoded to 2 matching capture; promote to GameConfig
when captures show variance.

504 tests pass (was 496; +8 spot-card-exchange tests).

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

SVSim.Database/SVSimDbContext.cs

@@ -76,6 +76,8 @@ public class SVSimDbContext : DbContext
     public DbSet<LeaderSkinShopSeriesEntry> LeaderSkinShopSeries => Set<LeaderSkinShopSeriesEntry>();
     public DbSet<LeaderSkinShopProductEntry> LeaderSkinShopProducts => Set<LeaderSkinShopProductEntry>();
     public DbSet<ViewerLeaderSkinSetClaim> ViewerLeaderSkinSetClaims => Set<ViewerLeaderSkinSetClaim>();
+    public DbSet<SpotCardExchangeEntry> SpotCardExchangeCatalog => Set<SpotCardExchangeEntry>();
+    public DbSet<ViewerSpotCardExchange> ViewerSpotCardExchanges => Set<ViewerSpotCardExchange>();
     public DbSet<MaintenanceCardEntry> MaintenanceCards => Set<MaintenanceCardEntry>();
     public DbSet<FeatureMaintenanceEntry> FeatureMaintenances => Set<FeatureMaintenanceEntry>();
     public DbSet<PreReleaseInfo> PreReleaseInfos => Set<PreReleaseInfo>();
@@ -209,6 +211,12 @@ public class SVSimDbContext : DbContext
             b.HasIndex(c => c.ViewerId);
         });
 
+        modelBuilder.Entity<ViewerSpotCardExchange>(b =>
+        {
+            b.HasKey(e => new { e.ViewerId, e.CardId });
+            b.HasIndex(e => e.ViewerId);
+        });
+
         modelBuilder.Entity<CardCosmeticReward>(b =>
         {
             b.HasKey(r => new { r.CardId, r.Type, r.CosmeticId });