Add ASP.NET Identity, AppUser, migrations

Introduce ASP.NET Core Identity with Guid keys: add Microsoft.AspNetCore.Identity.EntityFrameworkCore and update EF packages to 10.0.6. Replace DbContext with IdentityDbContext<AppUser, IdentityRole<Guid>, Guid>, apply entity configurations and map Identity tables to custom names (Users, Roles, UserRoles, etc.).

Add AppUser model (IsAdmin, IsActive, MustChangePassword, CreatedAt, UpdatedAt) and AppUserConfiguration to enforce required properties and table name. Add IdentitySeedService to create an initial admin account if none exists and log results.

Add generated migration InitIdentity and update the DbContext model snapshot. Wire up Identity in Program.cs (identity options, cookie config, AddEntityFrameworkStores), enable structured console logging and HTTP request logging, run migrations on startup and call the seed service, and enable authentication/authorization middleware. Update codexInfo.md to document the logging and seeding changes.

API/Database/Configurations/AppUserConfiguration.cs

@@ -0,0 +1,20 @@
+using API.Models;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Metadata.Builders;
+
+namespace API.Database.Configurations
+{
+    public class AppUserConfiguration : IEntityTypeConfiguration<AppUser>
+    {
+        public void Configure(EntityTypeBuilder<AppUser> builder)
+        {
+            builder.ToTable("Users");
+
+            builder.Property(x => x.CreatedAt).IsRequired();
+            builder.Property(x => x.UpdatedAt).IsRequired();
+            builder.Property(x => x.IsAdmin).IsRequired();
+            builder.Property(x => x.IsActive).IsRequired();
+            builder.Property(x => x.MustChangePassword).IsRequired();
+        }
+    }
+}