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.Api/appsettings.Development.json

@@ -6,7 +6,7 @@
     }
   },
   "ConnectionStrings": {
-    "DefaultConnection": "Server=(localdb)\\mssqllocaldb;Database=SlpModularCms;Trusted_Connection=True;MultipleActiveResultSets=true"
+    "DefaultConnection": "Server=127.0.0.1;Port=3306;Database=SlpModularCms;Uid=root;Pwd=<your-local-mariadb-password>"
   },
   "JwtSettings": {
     "Secret": "SuperSecretKeyForDevelopmentOnly_MustBeLongerThan32Bytes!",

src/SlpModularCms.Api/appsettings.json

@@ -10,7 +10,7 @@
   },
   "AllowedHosts": "*",
   "ConnectionStrings": {
-    "DefaultConnection": "Server=<production-db-host>;Database=SlpModularCms;User Id=<db-user>;Password=<db-password>;TrustServerCertificate=True"
+    "DefaultConnection": "Server=<production-db-host>;Port=3306;Database=SlpModularCms;Uid=<db-user>;Pwd=<db-password>"
   },
   "JwtSettings": {
     "Secret": "<secure-long-random-secret-key-from-env>",