Read stale pickled tagging-cache entries as misses, not 500s

[ajslater]

Problem

After upgrading to the 2.5.4 image, a user with online-tag matches pending review sees ModuleNotFoundError: No module named 'numpy' at startup (logged under "Clearing stale online tag scan marker on startup") and as 500s on /api/v4/admin/tag-prompts and /api/v4/admin/tag-sessions/snapshot.

Codex 2.5.3 (comicbox 5.2.2) scored covers with imagehash, whose hash distance is numpy.int64, so cover_score and the blended score were numpy.float64. serialize_candidate copied them into each pending prompt, which is pickled into the file-based tagging cache and kept across restarts. 2.5.4 (comicbox 5.3.1, which uses a pure-Python pHash) no longer installs numpy, so every read of those prompts fails while unpickling. ResilientFileBasedCache only treated zlib.error, UnpicklingError and EOFError as unreadable, so the error reached the caller.

Changes

• ResilientFileBasedCache also treats ImportError and AttributeError raised while loading an entry as unreadable: delete the file, report a miss. A pickle naming a module or class that no longer exists is as unloadable as a truncated one.
• The discard is logged as a warning (previously debug) with the cache key and the traceback. The file is gone afterwards, so the log is the only record, both for an admin whose review queue emptied and for debugging if this ever catches something it shouldn't.
• serialize_candidate casts score, metadata_score and cover_score to plain float, so a library's number type can't end up in the cache again.
• NEWS: new v2.5.5 section.

Prompts already stored with numpy scores are dropped on first read; those comics need another online-tag pass.

Testing

• New tests/test_resilient_cache.py saves a value whose class comes from a temporary module, then removes the module (ModuleNotFoundError) or just the class (AttributeError). It covers get, touch, the warning's content and the startup prompt prune. All of them failed before the fix with the same ModuleNotFoundError from Django's filebased.py that the user reported.
• New plain-float test in tests/test_onlinetag_serialize_candidate.py.
• make fix, make lint, make ty clean; make test: 1668 passed, 1 xfailed (Python), 994 passed (frontend).
• make lint still shows two existing reportUnreachable warnings in codex/views/browser/annotate/cover.py:63,69; this PR doesn't touch that file.

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