Add management command for auditing missing sources

[bjester]

Summary

• Adds CTE functionality to channel model
• Adds new management command that audits missing import sources for public channels, where source node was deleted or public channel was marked as deleted

References

#5683

Reviewer guidance

• Tests make sense?
• Can you run the command? It will generate a fix_missing_import_sources.csv in your CWD

LLMs

• LLMs were used to convert SQL queries into Django ORM queries, but the result was almost entirely rewritten
• Tests were bootstrapped then also rewritten

[rtibblesbot]

Audit command for missing import sources on public channels, with CTE support added to the Channel model. Code is clean and well-structured.

CI: linting and frontend builds pass; Python unit tests still pending. The "Update community pull requests spreadsheet" failure is an unrelated admin workflow.

• suggestion: Command name vs. current behavior mismatch (inline)
• suggestion: Duplicate CTE construction (inline)
• nitpick: .gitignore additions are unrelated to the PR scope but harmless

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@rtibblesbot's comments are generated by an LLM, and should be evaluated accordingly

How was this generated?

Reviewed the pull request diff checking for:

• Correctness: bugs, edge cases, undocumented behavior, resource leaks, hardcoded values
• Design: unnecessary complexity, naming, readability, comment accuracy, redundant state
• Architecture: duplicated concerns, minimal interfaces, composition over inheritance
• Testing: behavior-based assertions, mocks only at hard boundaries, accurate coverage
• Completeness: missing dependencies, unupdated usages, i18n, accessibility, security
• Principles: DRY (same reason to change), SRP, Rule of Three (no premature abstraction)
• Checked CI status and linked issue acceptance criteria
• For UI changes: inspected screenshots for layout, visual completeness, and consistency

[rtibblesbot]

praise: Good approach — inheriting from CTEQuerySet instead of models.QuerySet keeps the custom create/update_or_create methods intact while adding CTE support. The ChannelModelManager via from_queryset is the correct Django pattern for this.

[rtibbles]

I ran it locally and got this:

INFO:contentcuration.management.commands.fix_missing_import_sources:=== Iterating over private destination channels. ===
INFO:contentcuration.management.commands.fix_missing_import_sources:=== Done iterating over private destination channels. ===
INFO:contentcuration.management.commands.fix_missing_import_sources:Found 0 nodes across 0 channels.
INFO:contentcuration.management.commands.fix_missing_import_sources:Finished in 0.015901565551757812

Which seems accurate :)

Further testing we can do live.