fix(combat): remove plugin-side lifetime — backend accumulates now

The plugin's _lifetimeState was always identical to _sessionState
because both started fresh on every load/login and accumulated the
same events. Lifetime needs persistence across sessions.

Fix: plugin now only maintains _sessionState and sends lifetime=null.
The backend computes deltas between consecutive session snapshots and
accumulates them into a persisted lifetime in the combat_stats DB table.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

MosswartMassacre/CombatStatsTracker.cs

@@ -13,7 +13,7 @@ namespace MosswartMassacre
     ///
     /// Maintains two CombatSessionState objects:
     ///   - _sessionState: cleared on login / manual reset
-    ///   - _lifetimeState: accumulated across sessions, never cleared
+    ///   - Lifetime: accumulated on the backend by merging session deltas
     ///
     /// Every 10 seconds, sends a combat_stats snapshot to MosswartOverlord
     /// via WebSocket so the browser dashboard can display the same grid
@@ -25,7 +25,6 @@ namespace MosswartMassacre
 
         private readonly IPluginLogger _logger;
         private CombatSessionState _sessionState = new CombatSessionState();
-        private CombatSessionState _lifetimeState = new CombatSessionState();
         private System.Windows.Forms.Timer _sendTimer;
         private bool _dirty;
         private bool _disposed;
@@ -34,7 +33,6 @@ namespace MosswartMassacre
         {
             _logger = logger;
             _sessionState.SessionStart = DateTime.UtcNow.ToString("o");
-            _lifetimeState.SessionStart = DateTime.UtcNow.ToString("o");
         }
 
         public void Start()
@@ -198,7 +196,7 @@ namespace MosswartMassacre
             };
 
             apply(_sessionState);
-            apply(_lifetimeState);
+
             _dirty = true;
         }
 
@@ -219,7 +217,7 @@ namespace MosswartMassacre
             };
 
             apply(_sessionState);
-            apply(_lifetimeState);
+
             _dirty = true;
         }
 
@@ -235,7 +233,7 @@ namespace MosswartMassacre
             };
 
             apply(_sessionState);
-            apply(_lifetimeState);
+
             _dirty = true;
         }
 
@@ -251,7 +249,7 @@ namespace MosswartMassacre
             };
 
             apply(_sessionState);
-            apply(_lifetimeState);
+
             _dirty = true;
         }
 
@@ -267,7 +265,7 @@ namespace MosswartMassacre
             };
 
             apply(_sessionState);
-            apply(_lifetimeState);
+
             _dirty = true;
         }
 
@@ -541,7 +539,7 @@ namespace MosswartMassacre
                     character_name = SafeCharacterName(),
                     session_id = WebSocket.SessionId ?? "",
                     session = _sessionState,
-                    lifetime = _lifetimeState,
+                    lifetime = (object)null,  // lifetime is accumulated on the backend
                 };
                 _ = WebSocket.SendCombatStatsAsync(payload);
             }