Extend upload-nonce to author + late enrich tasks (fix stuck-at-50% on re-upload)

[vanajmoorthy]

Summary

PR #101 revoked the parent generate_reading_dna_task on re-upload, but the StoryGraph→Goodreads repro still hits the stuck-at-50% loading state. Diagnosed two uncovered paths:

1. check_author_mainstream_status_task had no nonce protection at all. StoryGraph uploads spawn one per new author at the two dispatch sites (core/services/dna_analyser.py:497, :652). Even after the parent DNA task is revoked, these keep running, flood the worker queue, and starve the new DNA task — visible as the loading bar freezing at the "Syncing books" stage (≈50%).
2. enrich_book_task only checked the nonce at task start. A task that passed the start check could still sit briefly on Book.objects.get before its ~5s of API calls. If a new upload landed in that window, the in-flight task would still spend the API time and write stale data.

Changes

• check_author_mainstream_status_task now accepts optional user_id + upload_nonce; exits early on nonce mismatch using the same safe_cache_get("upload_nonce_{user_id}") pattern as enrich_book_task.
• enrich_book_task keeps its start-of-task check and adds a second re-check after Book.objects.get, before invoking enrich_book_from_apis.
• Both dispatch sites in core/services/dna_analyser.py now pass user_id=upload_user_id, upload_nonce=upload_nonce.
• Three new tests for the author-task path (skip / run / backwards-compat with no nonce kwargs) and one for the mid-task re-check on enrich_book_task.

Out of scope (follow-up)

• The deeper "in-flight enrich task finishes its book.save() even when superseded mid-API-call" window. Closing this requires splitting enrich_book_from_apis into fetch + persist phases so the task can re-check between them. Worth doing if we see stale writes in prod, but the API window is short (~5s per book, single-worker) compared to the queue-depth issue this PR addresses.
• A higher-priority queue for generate_reading_dna_task so it can't be starved by subtask backlog. Deployment-affecting change; left for a separate decision.

Test plan

• core.tests.test_integration.CheckAuthorMainstreamStatusTaskTests — skip / run / no-nonce.
• core.tests.test_integration.BookEnrichmentIntegrationTests.test_enrich_book_task_skipped_at_second_check_if_nonce_changed_during_db_fetch — validates the new mid-task re-check by mutating the nonce inside a Book.objects.get side-effect.
• Full suite (351 tests) passes.
• Manual: on prod, repro the StoryGraph→Goodreads flow and verify the loading bar progresses past 50%.