feat(items): add Imperial Sword (0/50) and Imperial Staff (5/50)

Two new weapons placed at free item slots so they never collide with
standard Season 6 items. The client name DB (Item_eng.bmd) is the source
of tooltip name/stats, so only free slots are usable for custom items.

Both seed paths are covered:
- Fresh DB: Weapons.Initialize() calls the new CreateImperialWeapons050().
- Existing DB: AddImperialWeapons050UpdateSeason6 (UpdateVersion 95),
  applied from the AdminPanel Updates page, with an idempotency guard.

Imperial Sword is a Magic Gladiator weapon (copy of 0/28), Imperial Staff
is a Wizard weapon (copy of 5/31).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/Persistence/Initialization/Updates/UpdateVersion.cs

@@ -479,4 +479,9 @@ public enum UpdateVersion
     /// The version of the <see cref="AddIsQuestItemFlagPlugIn"/>.
     /// </summary>
     AddIsQuestItemFlag = 94,
+
+    /// <summary>
+    /// The version of the <see cref="AddImperialWeapons050UpdateSeason6"/>.
+    /// </summary>
+    AddImperialWeapons050Season6 = 95,
 }