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/database/dbmem/dbmem.go

@@ -23,11 +23,9 @@ import (
 	"golang.org/x/exp/maps"
 	"golang.org/x/xerrors"
 
-	"github.com/coder/coder/v2/coderd/notifications/types"
-	"github.com/coder/coder/v2/coderd/prebuilds"
-
 	"github.com/coder/coder/v2/coderd/database"
 	"github.com/coder/coder/v2/coderd/database/dbtime"
+	"github.com/coder/coder/v2/coderd/notifications/types"
 	"github.com/coder/coder/v2/coderd/rbac"
 	"github.com/coder/coder/v2/coderd/rbac/regosql"
 	"github.com/coder/coder/v2/coderd/util/slice"
@@ -160,7 +158,7 @@ func New() database.Store {
 	q.mutex.Lock()
 	// We can't insert this user using the interface, because it's a system user.
 	q.data.users = append(q.data.users, database.User{
-		ID:             prebuilds.SystemUserID,
+		ID:             database.PrebuildsSystemUserID,
 		Email:          "prebuilds@coder.com",
 		Username:       "prebuilds",
 		CreatedAt:      dbtime.Now(),