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.Core.Tests/Hosting/DatabaseMigrationExtensionsTests.cs

@@ -1,5 +1,5 @@
 using FluentAssertions;
-using Microsoft.Data.SqlClient;
+using MySql.Data.MySqlClient;
 using SlpModularCms.Core.Hosting;
 using Xunit;
 
@@ -47,9 +47,9 @@ public class DatabaseMigrationExtensionsTests
 
     public static TheoryData<Exception> TransientFailures() =>
     [
-        MakeSqlException(),
+        MakeMySqlException(),
         new TimeoutException("Connect Timeout expired."),
-        new InvalidOperationException("wrapper", MakeSqlException()),
+        new InvalidOperationException("wrapper", MakeMySqlException()),
     ];
 
     public static TheoryData<Exception> NonTransientFailures() =>
@@ -71,18 +71,18 @@ public class DatabaseMigrationExtensionsTests
     }
 
     /// <summary>
-    /// <see cref="SqlException"/> has no public constructor, so one is produced through the
+    /// <see cref="MySqlException"/> has no public constructor, so one is produced through the
     /// framework's own factory path via reflection.
     /// </summary>
-    private static Exception MakeSqlException()
+    private static Exception MakeMySqlException()
     {
         try
         {
             // Deliberately unreachable host and a very short timeout: this genuinely produces a
-            // SqlException rather than a hand-built stand-in, so the classifier is tested against
+            // MySqlException rather than a hand-built stand-in, so the classifier is tested against
             // the real type it will encounter in production.
-            using var connection = new SqlConnection(
-                "Server=localhost,9;Database=none;User Id=sa;Password=none;Connect Timeout=1;TrustServerCertificate=True");
+            using var connection = new MySqlConnection(
+                "Server=localhost;Port=9;Database=none;Uid=none;Pwd=none;Connection Timeout=1");
             connection.Open();
         }
         catch (Exception ex)