Revoke prior DNA task on re-upload (fix stuck-at-50%)

[vanajmoorthy]

Summary

Fixes the "stuck at 50%" bug from PR #98's handoff doc (§2): uploading a CSV while a previous DNA task was still running caused the new task's progress bar to stall at ~50% until the old task finished.

Root cause: Both tasks ran concurrently and contended on Postgres row locks during `get_or_create` on shared Book/Author rows. The new task blocked on the old task's open transaction, so the dashboard's polling saw frozen progress.

Fix: Before dispatching the new DNA task, `upload_view` revokes the prior pending task (`AsyncResult.revoke(terminate=True, signal="SIGTERM")`) — best-effort, with a try/except so a failed revoke can't block the new upload. This matches user intent ("I clearly meant to replace that upload"), which the handoff doc preferred over a reject-while-pending guard.

Why no orphan-row cleanup is needed:

• `calculate_full_dna` already deletes `UserBook` rows not in the current upload (`core/services/dna_analyser.py:687`).
• Orphan `Book` and `Author` rows are shared resources — nothing depends on them being scoped to a single user's upload.
• The existing upload-nonce mechanism makes any in-flight `enrich_book_task` from the revoked upload exit early.

Side fix: Switched `pending_dna_task_id` write to `UserProfile.objects.filter(...).update(...)`. The previous code accessed `request.user.userprofile` after `.delay()`, which in eager-mode tests fetched fresh after the task wrote DNA. My new revoke logic accesses `userprofile` before dispatch, caching a stale instance whose later `.save()` clobbered the DNA. `.update()` sidesteps the cached instance entirely.

Test plan

• `test_upload_revokes_prior_pending_task` — prior task ready=False → revoke called with terminate=True, SIGTERM
• `test_upload_does_not_revoke_completed_task` — prior task ready=True → revoke not called
• `test_upload_succeeds_when_revoke_raises` — revoke raises → upload still succeeds, new task ID written
• `test_reupload_clears_stale_userbooks` — re-upload with different books removes the old UserBook rows
• Full suite: `docker-compose -f docker-compose.local.yml exec web poetry run python manage.py test` → 343 passed
• Manual: upload a large CSV, immediately re-upload before the first finishes; confirm the second progress bar advances normally instead of stalling