Add project files.

Database/Entities/PuzzleCategory.cs

@@ -0,0 +1,35 @@
+namespace ConnectionsAPI.Database.Entities
+{
+    public class PuzzleCategory
+    {
+        /// <summary>
+        /// Primary key of the entity
+        /// </summary>
+        public int Id { get; set; }
+
+        /// <summary>
+        /// The name of the category in this Connections puzzle
+        /// </summary>
+        public string Name { get; set; } = string.Empty;
+
+        /// <summary>
+        /// The color of the category in this Connections puzzle; Also used for sorting
+        /// </summary>
+        public PuzzleCategoryColor Color { get; set; }
+
+        /// <summary>
+        /// The ID of the associated Connections puzzle
+        /// </summary>
+        public int PuzzleId { get; set; }
+        
+        /// <summary>
+        /// The associated puzzle instance
+        /// </summary>
+        public virtual Puzzle? Puzzle { get; set; }
+
+        /// <summary>
+        /// The cards associated with this category
+        /// </summary>
+        public ICollection<PuzzleCard> PuzzleCards { get; set; } = [];
+    }
+}