Adds a shared-harness EXTRA_ENV mechanism (recipe_meta.py dict or domain-callable),
applied in deploy_app at every deploy path — no per-recipe harness surgery (D5).
cryptpad uses it for its required distinct SANDBOX_DOMAIN. Tests assert data
survival via a marker file in the backed-up cryptpad_data volume (exec_in_app,
since cryptpad data isn't HTTP-served).
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
