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/modelmethods.go

@@ -229,6 +229,24 @@ func (w Workspace) RBACObject() rbac.Object {
 	return w.WorkspaceTable().RBACObject()
 }
 
+// IsPrebuild returns true if the workspace is a prebuild workspace.
+// A workspace is considered a prebuild if its owner is the prebuild system user.
+func (w Workspace) IsPrebuild() bool {
+	return w.OwnerID == PrebuildsSystemUserID
+}
+
+// AsPrebuild returns the RBAC object corresponding to the workspace type.
+// If the workspace is a prebuild, it returns a prebuilt_workspace RBAC object.
+// Otherwise, it returns a normal workspace RBAC object.
+func (w Workspace) AsPrebuild() rbac.Object {
+	if w.IsPrebuild() {
+		return rbac.ResourcePrebuiltWorkspace.WithID(w.ID).
+			InOrg(w.OrganizationID).
+			WithOwner(w.OwnerID.String())
+	}
+	return w.RBACObject()
+}
+
 func (w WorkspaceTable) RBACObject() rbac.Object {
 	if w.DormantAt.Valid {
 		return w.DormantRBAC()
@@ -246,6 +264,24 @@ func (w WorkspaceTable) DormantRBAC() rbac.Object {
 		WithOwner(w.OwnerID.String())
 }
 
+// IsPrebuild returns true if the workspace is a prebuild workspace.
+// A workspace is considered a prebuild if its owner is the prebuild system user.
+func (w WorkspaceTable) IsPrebuild() bool {
+	return w.OwnerID == PrebuildsSystemUserID
+}
+
+// AsPrebuild returns the RBAC object corresponding to the workspace type.
+// If the workspace is a prebuild, it returns a prebuilt_workspace RBAC object.
+// Otherwise, it returns a normal workspace RBAC object.
+func (w WorkspaceTable) AsPrebuild() rbac.Object {
+	if w.IsPrebuild() {
+		return rbac.ResourcePrebuiltWorkspace.WithID(w.ID).
+			InOrg(w.OrganizationID).
+			WithOwner(w.OwnerID.String())
+	}
+	return w.RBACObject()
+}
+
 func (m OrganizationMember) RBACObject() rbac.Object {
 	return rbac.ResourceOrganizationMember.
 		WithID(m.UserID).