Configure exe sandbox templates with the existing Drukbox contract

[czpython]

What changed

Expose the registry values that the existing exe provider needs to publish sandbox templates. Fresh exe installs now show blank EXE_IMAGE_REGISTRY, EXE_REGISTRY_USERNAME, and EXE_REGISTRY_PASSWORD entries in [sandbox.exe]. The existing renderer passes configured values through unchanged. One full repository holds all templates.

Send readable app/script labels through the existing template API; template reuse remains based on content. No public documentation changes.

Risk

This PR works with the current Drukbox contract. It has no dependency on the future Drukbox registry changes. Operators must supply the registry values and recreate Drukbox with its existing image. The PR alone does not configure production credentials or rebuild an existing failed template.

No database migration, SDK change, or Drukbox image update. The future drafts are Drukbox czpython/drukbox#28 and Druks #434. Neither blocks this PR.

To unblock the deployment, set these values in the existing [sandbox.exe] table:

EXE_IMAGE_REGISTRY = "ghcr.io/<owner>/<repository>"
EXE_REGISTRY_USERNAME = "<username>"
EXE_REGISTRY_PASSWORD = "<registry-token>"

Render .env, recreate Drukbox with its current image, delete the failed template through its API, then run druks doctor.

Verification

• uv run ruff check backend: passed.
• uv run ruff format --check backend: passed.
• uv run pytest backend/tests/test_setup_env.py backend/tests/test_declared_sandboxes.py -q: 46 passed.
• Installed the proof app with uv pip install -e backend/tests/druks-field_notes before the full backend run.
• uv run pytest backend/ -q: 1,584 passed, 19 warnings.
• The final edit only removes documentation. Backend code is unchanged from the tested commit.
• No frontend files changed. No local frontend checks or live registry/VM deployment tests were run.

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Project	Status	Preview	Updated
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