azure: firewall rules and subnets around postgres server

[Brandel-T]

Add

• firewal rules
• subnets
• network security groups, etc.

to securely access the postgres db, and

• connect it with the backend webapp

See:

• https://registry.terraform.io/providers/hashicorp/azurerm/latest/docs/resources/postgresql_flexible_server
• https://learn.microsoft.com/de-de/azure/developer/terraform/deploy-postgresql-flexible-server-database?tabs=azure-cli