Move model configuration to its own classes

Jellyfin.Server.Implementations/ModelConfiguration/PermissionConfiguration.cs

@@ -0,0 +1,26 @@
+using Jellyfin.Data.Entities;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+
+namespace Jellyfin.Server.Implementations.ModelConfiguration
+{
+    /// <summary>
+    /// FluentAPI configuration for the Permission entity.
+    /// </summary>
+    public class PermissionConfiguration : IEntityTypeConfiguration<Permission>
+    {
+        /// <inheritdoc/>
+        public void Configure(EntityTypeBuilder<Permission> builder)
+        {
+            // Indexes
+
+            // Used to get a user's permissions or a specific permission for a user.
+            // Also prevents multiple values being created for a user.
+            // Filtered over non-null user ids for when other entities (groups, API keys) get permissions
+            builder
+                .HasIndex(p => new { p.UserId, p.Kind })
+                .HasFilter("[UserId] IS NOT NULL")
+                .IsUnique();
+        }
+    }
+}