Add skip_pre_post build option, cancel endpoint, and recipe package

- skip_pre_post flag on builds bypasses apt update/clean pre/post steps for
  faster iteration when the recipe handles its own environment setup
- POST /v1/admin/builds/{id}/cancel endpoint marks an in-progress build as
  cancelled; UpdateBuildStatus now also sets completed_at for 'cancelled'
- internal/recipe: typed recipe parser and executor (RUN/ENV/COPY steps)
  replacing the raw string slice approach in the build worker
- pre/post build commands prefixed with RUN to match recipe step format

frontend/src/lib/api/builds.ts

@@ -38,6 +38,7 @@ export type CreateBuildParams = {
 	healthcheck?: string;
 	vcpus?: number;
 	memory_mb?: number;
+	skip_pre_post?: boolean;
 };
 
 export async function createBuild(params: CreateBuildParams): Promise<ApiResult<Build>> {
@@ -69,3 +70,7 @@ export async function listAdminTemplates(): Promise<ApiResult<AdminTemplate[]>>
 export async function deleteAdminTemplate(name: string): Promise<ApiResult<void>> {
 	return apiFetch('DELETE', `/api/v1/admin/templates/${name}`);
 }
+
+export async function cancelBuild(id: string): Promise<ApiResult<void>> {
+	return apiFetch('POST', `/api/v1/admin/builds/${id}/cancel`);
+}