Mirror SCORM zips to course contentstore for OLX export/import

[Syed-Ali-Abbas-568]

Summary

Open edX's OLX export only bundles assets stored in the course contentstore, but SCORM zips are unpacked into Django's default_storage. As a result, an exported course re-imported into a new course keeps the SCORM block's metadata but the package fails to play — the unpacked files are absent from the new course's storage.

This PR adds an opt-in path that mirrors each uploaded zip into the course contentstore (which IS bundled in OLX exports) and rehydrates the unpacked tree from the contentstore on first access after an import. It composes cleanly with the per-block extraction path from PR #71 and is disabled by default — legacy behavior is unchanged unless an operator opts in.

Fixes #108.

What changed

• On upload (studio_submit): mirror the original zip into the current course's contentstore at scorm_packages/<sha1>.zip (locked).
• On read (index_page_url, assets_proxy): if package_meta is set but the extracted tree is missing from default_storage, fetch the zip from the contentstore and re-extract.
• GC on upload: sweep scorm_packages/<sha1>.zip assets that no SCORM block on either the draft or published branch references, and delete them. Caps the contentstore at ≤1 published + ≤1 draft zip per block.
• All contentstore calls are wrapped in try/except and xmodule imports are lazy, so uploads still succeed in environments where the contentstore is unavailable (tests, non-platform installs).

How to enable

XBLOCK_SETTINGS["ScormXBlock"] = {
    "CONTENTSTORE_SYNC_ENABLED": True,
}

[Danyal-Faheem]

@ahmed-arb Thoughts on keeping this turned off by default?

[Danyal-Faheem]

nit: lets try to avoid nested try excepts

[Danyal-Faheem]

This is a really cool approach. Lets also document this for future reference.

[Danyal-Faheem]

Exceptional work! I just have a few minor comments.

[ahmed-arb]

Thanks for tackling this @Syed-Ali-Abbas-568. The implementation here is very clever.

That said, I want to raise an architectural concern before this merges. The fix asks every site operator to opt into a permanent, ongoing cost (two writes per upload, a read-time check on every page load, a GC sweep on every save, plus the risk of default_storage and the contentstore drifting out of sync) in order to make a single, infrequent event (OLX export/import) work correctly. And operators have to know to flip the flag before they ever need to export. In practice many won't realize they needed it until an export silently produces a broken course on re-import, which is the same failure mode #108 describes today, just with an extra step in the postmortem.

I think we're patching the wrong layer. Open edX's OLX exporter is the component that knows it's bundling a course and walks its blocks, so that's the natural place to be SCORM-aware. If the exporter pulled the package out of default_storage and bundled it at export time (and restored it to default_storage on import), operators wouldn't need to configure anything, the cost would be paid only at export where it belongs, and there'd be no continuous mirror to keep consistent. The xblock would stay a pure xblock and not have to reach into modulestore/contentstore internals.

I realize that's a heavier lift because it requires changes in edx-platform rather than a self-contained xblock change, and this PR is something that can ship today. But before we commit to a continuous-sync code path here as the long-term answer, has anyone raised this with Open edX core? Framed as "the platform's export pipeline doesn't fully support an xblock that ships in the standard distribution," it seems like the kind of thing that would get traction upstream. At minimum I'd want that discussion to have happened before we lock this in. Even if we end up merging this as a stopgap, knowing whether it's a stopgap or the permanent answer matters for what we're willing to live with.