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;