Add Notifications Service - 5 endpoints, 75/73+ complete

- NotificationsController.cs with 5 endpoints:
  * GET /synergy/notifications - list with unreadOnly filter
  * GET /synergy/notifications/unread-count - badge count
  * POST /synergy/notifications/mark-read - single or bulk
  * POST /synergy/notifications/send - player or broadcast
  * DELETE /synergy/notifications/{id} - delete by player
- Notification entity added to RR3DbContext (FK to Users, cascade delete)
- NotificationDto, NotificationsResponse, MarkReadRequest,
  SendNotificationRequest models added to ApiModels.cs
- Migration AddNotificationsTable applied
- Build: 0 errors, 0 warnings
- All 5 endpoints tested and working

Server now: 75/73 core endpoints (notifications + admin delete = 76)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

RR3CommunityServer/Models/ApiModels.cs

@@ -328,3 +328,43 @@ public class RecordSubmissionResponse
     public int GoldEarned { get; set; }
     public int CashEarned { get; set; }
 }
+
+// ==================== NOTIFICATIONS ====================
+
+public class NotificationDto
+{
+    public int Id { get; set; }
+    public string Type { get; set; } = string.Empty;
+    public string Title { get; set; } = string.Empty;
+    public string Message { get; set; } = string.Empty;
+    public bool IsRead { get; set; }
+    public long CreatedAt { get; set; } // Unix timestamp
+    public long? ExpiresAt { get; set; }
+}
+
+public class NotificationsResponse
+{
+    public List<NotificationDto> Notifications { get; set; } = new();
+    public int TotalCount { get; set; }
+    public int UnreadCount { get; set; }
+}
+
+public class UnreadCountResponse
+{
+    public int UnreadCount { get; set; }
+}
+
+public class MarkReadRequest
+{
+    public string SynergyId { get; set; } = string.Empty;
+    public List<int>? NotificationIds { get; set; } // null = mark all read
+}
+
+public class SendNotificationRequest
+{
+    public string? SynergyId { get; set; } // null = send to all players
+    public string Type { get; set; } = "system";
+    public string Title { get; set; } = string.Empty;
+    public string Message { get; set; } = string.Empty;
+    public int? ExpiresInHours { get; set; } // null = never expires
+}