terraform: module has no machines set for placement

## Summary

The Terraform module supports placing PostgreSQL on a single pre-created machine via the
`machine` variable, but there is no way to place an HA (multi-unit) deployment across a set
of machines. `machine` is a single string, so explicit placement and HA are mutually
exclusive today.

## Details

At `v16/1.305.0`, `terraform/main.tf`:

```hcl
machines = var.machine != null ? [var.machine] : null
units = var.machine == null ? var.units : null
```

`variable "machine"` is described as *"Target Juju machine to deploy on"*. So a 3-unit
cluster placed on three `juju_machine` resources is not expressible — you get either
placement with one unit, or three units with provisioner-allocated machines.

This came out of #911, which asked for simplified single-VM placement; that was implemented
as the singular `machine`. This issue is the HA follow-up.

The `juju_application` resource itself already takes a set:

> `machines` (Set of String) Specify the target machines for the application's units. The
> number of machines in the set indicates the unit count for the application.

## Impact

`canonical/landscape-server-operator`'s `landscape-scalable` product module colocates its
stack on a fixed set of machines, and postgresql is one of the applications it cannot
place, alongside haproxy. Its source carries the comment:

```hcl
# machines is not supported by the external postgresql module (v16/1.165.0)
```

## Request

Add a `machines` (set of string) variable alongside the existing `machine`, e.g.:

```hcl
machines = var.machines != null ? var.machines : (var.machine != null ? [var.machine] : null)
units = (var.machines == null && var.machine == null) ? var.units : null
```

keeping `machine` working for the single-VM case from #911.