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

db/migrations/20260330150223_build_options.sql

@@ -0,0 +1,11 @@
+-- +goose Up
+
+-- Allow completed_at to be set when a build is cancelled.
+-- (The UpdateBuildStatus query is updated in sqlc; no schema change needed for that.)
+
+-- Add skip_pre_post flag: when true, the pre-build and post-build command phases
+-- are skipped for this build.
+ALTER TABLE template_builds ADD COLUMN skip_pre_post BOOLEAN NOT NULL DEFAULT FALSE;
+
+-- +goose Down
+ALTER TABLE template_builds DROP COLUMN skip_pre_post;

db/queries/template_builds.sql

@@ -1,6 +1,6 @@
 -- name: InsertTemplateBuild :one
-INSERT INTO template_builds (id, name, base_template, recipe, healthcheck, vcpus, memory_mb, status, total_steps, template_id, team_id)
-VALUES ($1, $2, $3, $4, $5, $6, $7, 'pending', $8, $9, $10)
+INSERT INTO template_builds (id, name, base_template, recipe, healthcheck, vcpus, memory_mb, status, total_steps, template_id, team_id, skip_pre_post)
+VALUES ($1, $2, $3, $4, $5, $6, $7, 'pending', $8, $9, $10, $11)
 RETURNING *;
 
 -- name: GetTemplateBuild :one
@@ -12,8 +12,8 @@ SELECT * FROM template_builds ORDER BY created_at DESC;
 -- name: UpdateBuildStatus :one
 UPDATE template_builds
 SET status = $2,
-    started_at = CASE WHEN $2 = 'running' AND started_at IS NULL THEN NOW() ELSE started_at END,
-    completed_at = CASE WHEN $2 IN ('success', 'failed') THEN NOW() ELSE completed_at END
+    started_at   = CASE WHEN $2 = 'running'   AND started_at   IS NULL THEN NOW() ELSE started_at   END,
+    completed_at = CASE WHEN $2 IN ('success', 'failed', 'cancelled') THEN NOW() ELSE completed_at END
 WHERE id = $1
 RETURNING *;