Switches the database from SQL Server to MariaDB

The target Pi only has MariaDB, and SQL Server has no ARM64 build at
all - not a config problem, a real gap discovered during deployment
setup. Swapped the EF Core provider, regenerated every migration,
updated connection strings and the backup script everywhere they
appear.

Took two tries to land on a provider that actually works: Pomelo
builds fine against this project's EF Core 10 packages but fails at
runtime (it's compiled against 9's internal API surface, which moved
in 10 wherever Identity/DataProtection force the newer packages).
Oracle's official provider builds and migrates fine but has a real
MariaDB bug in its own migration-lock code, reproduced against a live
database. Kept Oracle's provider and worked around just that one
broken method - everything else it does is correct - rather than
give up more of the stack to chase a workaround.

Verified against a real local MariaDB end to end: all three
migrations applied, both hosts start clean, full suite still green.

src/SlpModularCms.Modules.Master/MasterModule.cs

@@ -1,10 +1,12 @@
 using System.Diagnostics.CodeAnalysis;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Migrations;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Http.Resilience;
 using Polly;
+using SlpModularCms.Core.Hosting;
 using SlpModularCms.Core.Modules;
 using SlpModularCms.Modules.Master.BackgroundServices;
 using SlpModularCms.Modules.Master.Data;
@@ -33,7 +35,8 @@ public class MasterModule : IModule
         services.AddDbContext<MasterDbContext>((serviceProvider, options) =>
         {
             var config = serviceProvider.GetRequiredService<IConfiguration>();
-            options.UseSqlServer(config.GetConnectionString("DefaultConnection"));
+            options.UseMySQL(config.GetConnectionString("DefaultConnection")!);
+            options.ReplaceService<IHistoryRepository, NonLockingMySQLHistoryRepository>();
         });
 
         services.AddScoped<ICmsInstanceRepository, CmsInstanceRepository>();