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.

aidlc-docs/features/gitea-deployment-workflow/operations/deployment/rollback-plan.md

@@ -46,8 +46,7 @@ A backup is taken before every **production** deploy (`operations/deployment/dep
 § 4, invoked by `deploy-scp.yaml` when `run_db_backup: true`). To restore:
 
 ```bash
-sqlcmd -S <server> -U <user> -P <password> -C -Q \
-  "RESTORE DATABASE [SlpModularCmsProduction] FROM DISK = N'<path-to-backup>.bak' WITH REPLACE"
+gunzip -c <path-to-backup>.sql.gz | mariadb -h <server> -u <user> -p SlpSoftwareProduction
 ```
 
 Restoring a database backup and rolling back the application release are **independent actions** —