feat(inventory): add GrantSource enum + message lookup

Introduces GrantSource (17 values, DB-persisted) and GrantSourceMessages.For()
for the item-acquire-history feature. Values 1/2 mirror prod captures;
coverage test verifies every enum value has a non-empty message.

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

SVSim.UnitTests/Services/Inventory/InventoryHistoryTests.cs

@@ -0,0 +1,26 @@
+using SVSim.Database.Services.Inventory;
+
+namespace SVSim.UnitTests.Services.Inventory;
+
+public class InventoryHistoryTests
+{
+    [Test]
+    public void GrantSourceMessages_returns_known_messages_for_seeded_sources()
+    {
+        Assert.That(GrantSourceMessages.For(GrantSource.DailyBonus), Is.EqualTo("Daily Bonus"));
+        Assert.That(GrantSourceMessages.For(GrantSource.PackOpen), Is.EqualTo("From buying card packs"));
+        Assert.That(GrantSourceMessages.For(GrantSource.CardCosmeticCascade), Is.EqualTo("Card cosmetic"));
+        Assert.That(GrantSourceMessages.For(GrantSource.Unknown), Is.EqualTo("Unknown"));
+    }
+
+    [Test]
+    public void GrantSourceMessages_covers_every_enum_value()
+    {
+        foreach (GrantSource source in Enum.GetValues<GrantSource>())
+        {
+            var message = GrantSourceMessages.For(source);
+            Assert.That(message, Is.Not.Null.And.Not.Empty,
+                $"GrantSource.{source} has no message defined.");
+        }
+    }
+}