docs(inventory): annotate GrantSource gap + For() exception

Add comment above AdminGrant = 99 explaining the intentional 16–98 gap,
and add <exception> XML doc on GrantSourceMessages.For() so IDE tooling
surfaces the ArgumentOutOfRangeException.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.Database/Services/Inventory/GrantSource.cs

@@ -28,6 +28,7 @@ public enum GrantSource
     AchievementReward = 13,
     SerialCodeRedeem = 14,
     CardCosmeticCascade = 15,
+    // Reserved high to stay visually distinct from gameplay sources; 16–98 are intentionally unused.
     AdminGrant = 99,
 }
 
@@ -37,6 +38,7 @@ public enum GrantSource
 /// </summary>
 public static class GrantSourceMessages
 {
+    /// <exception cref="ArgumentOutOfRangeException">An unmapped <see cref="GrantSource"/> value was passed.</exception>
     public static string For(GrantSource source) => source switch
     {
         GrantSource.Unknown             => "Unknown",