Add project files.

Database/ConnectionsContext.cs

@@ -0,0 +1,18 @@
+using ConnectionsAPI.Database.Entities;
+using Microsoft.EntityFrameworkCore;
+
+namespace ConnectionsAPI.Database
+{
+    public class ConnectionsContext(DbContextOptions<ConnectionsContext> dbContextOptions) : DbContext(dbContextOptions)
+    {
+        public DbSet<Puzzle> Puzzles { get; set; }
+
+        protected override void OnModelCreating(ModelBuilder modelBuilder)
+        {
+            modelBuilder.Entity<Puzzle>()
+                .HasIndex(x => x.PrintDate).IsUnique();
+
+            base.OnModelCreating(modelBuilder);
+        }
+    }
+}