Store original uploaded filename (admin-only) for talks/posters/pubs (#1391)

[jonfroehlich]

Closes #1391.

What & why

Artifact.save() renames uploaded files to the standardized Author_TitleInTitleCase_VenueYear scheme, destroying the one human-recognizable clue in the upload name (e.g. MyTalk_v3_final.pptx). This stores that original name in two new admin-only fields and surfaces it read-only on the change form as a provenance breadcrumb.

Changes

• Artifact (abstract base): add nullable, editable=False original_pdf_filename / original_raw_filename. A capture block at the top of save() snapshots the basename only on a genuine new upload — the first save, or an edit where the file field is in the incoming update_fields (before the rename logic appends to it). It does not fire on the m2m authors_changed rename re-save or on metadata-only edits, so a standardized name never clobbers a stored original.
• ArtifactAdmin: read-only "Originally uploaded as" row injected into the Files fieldset on the change form (inherited by Talk/Poster/Publication).
• backfill_original_filenames command (new): recovers originals for the many bulk-imported prod rows whose files were never renamed (their on-disk name still is the original). If a file's basename ≠ generate_filename() and isn't a -<timestamp> uniquified variant of it, the current basename is recorded as the original. Idempotent (fills nulls only); wired into docker-entrypoint.sh (step 4.7b).
• Tests: capture-survives-rename, edit-replace, metadata-only-no-clobber, backfill recover/skip/idempotent/uniquified, admin display. Full suite green (551 tests).

Why the backfill matters / ordering

Investigating the prod dump: the unrenamed files are genuine originals (title-derived names + Django's _xxxxxxx collision suffix) from rows that never went through an authored save(); the historical rename one-shots are commented out of the entrypoint. Latent risk: editing any legacy row in admin (even metadata) triggers the rename and destroys its original name — and the in-save() capture won't fire on a metadata edit. The backfill is the only thing that preserves these, so it runs at container start before admin traffic. Do not re-enable any rename one-shot until this has deployed and backfilled. (Re-standardizing those legacy files is a separate follow-up issue.)

Migration

No committed migration (matches repo convention): settings_test builds tables from models; prod/test run makemigrations website at container start, generating the two additive nullable columns.

Notes

• UI: the only UI surface is a read-only text row on the admin change form (login-gated, not in the Pa11y public scan set). Admin before/after screenshot to be added.
• No version bump in this PR (leave for the release cut).

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