feat: add MySQL/MariaDB support

[cl8dep]

Summary

Add MySQL/MariaDB as a supported database engine alongside PostgreSQL.

Implementation Notes

• Add Pomelo.EntityFrameworkCore.MySql NuGet package to Piro.Infrastructure
• Update InfrastructureServiceExtensions to detect a mysql:// connection string prefix and configure UseMySql
• Update Quartz to use its MySQL ADO.NET store when MySQL is configured
• Generate a separate migration set under Migrations/MySQL/ using --output-dir so MySQL-specific SQL doesn't conflict with the PostgreSQL migrations
• Update DbContext resolution to point at the correct migrations assembly based on the active provider
• Update documentation / docker-compose.yml example with a MySQL variant

Related

• Currently only PostgreSQL is supported after removing SQLite in the feat/serilog-logs branch