feat: allow TemplateAdmin to delete prebuilds via auth layer (#18333)

## Description

This PR adds support for deleting prebuilt workspaces via the
authorization layer. It introduces special-case handling to ensure that
`prebuilt_workspace` permissions are evaluated when attempting to delete
a prebuilt workspace, falling back to the standard `workspace` resource
as needed.

Prebuilt workspaces are a subset of workspaces, identified by having
`owner_id` set to `PREBUILD_SYSTEM_USER`.
This means:
* A user with `prebuilt_workspace.delete` permission is allowed to
**delete only prebuilt workspaces**.
* A user with `workspace.delete` permission can **delete both normal and
prebuilt workspaces**.

⚠️ This implementation is scoped to **deletion operations only**. No
other operations are currently supported for the `prebuilt_workspace`
resource.

To delete a workspace, users must have the following permissions:
* `workspace.read`: to read the current workspace state
* `update`: to modify workspace metadata and related resources during
deletion (e.g., updating the `deleted` field in the database)
* `delete`: to perform the actual deletion of the workspace

## Changes

* Introduced `authorizeWorkspace()` helper to handle prebuilt workspace
authorization logic.
* Ensured both `prebuilt_workspace` and `workspace` permissions are
checked.
* Added comments to clarify the current behavior and limitations.
* Moved `SystemUserID` constant from the `prebuilds` package to the
`database` package `PrebuildsSystemUserID` to resolve an import cycle
(commit
https://github.com/coder/coder/pull/18333/commits/f24e4ab4b6f0a56726fd04be2d7302c9fdb52d53).
* Update middleware `ExtractOrganizationMember` to include system user
members.

coderd/wsbuilder/wsbuilder.go

@@ -918,7 +918,18 @@ func (b *Builder) authorize(authFunc func(action policy.Action, object rbac.Obje
 		msg := fmt.Sprintf("Transition %q not supported.", b.trans)
 		return BuildError{http.StatusBadRequest, msg, xerrors.New(msg)}
 	}
-	if !authFunc(action, b.workspace) {
+
+	// Special handling for prebuilt workspace deletion
+	authorized := false
+	if action == policy.ActionDelete && b.workspace.IsPrebuild() && authFunc(action, b.workspace.AsPrebuild()) {
+		authorized = true
+	}
+	// Fallback to default authorization
+	if !authorized && authFunc(action, b.workspace) {
+		authorized = true
+	}
+
+	if !authorized {
 		if authFunc(policy.ActionRead, b.workspace) {
 			// If the user can read the workspace, but not delete/create/update. Show
 			// a more helpful error. They are allowed to know the workspace exists.