Decode+encode is a few ms (the encoder downscales before its pixel loops), and the
tee fires on Close after the response is fully written, so the hash can be computed
in the serving goroutine: the queue, wake/stop lifecycle, lazy-start admin context,
and the e2e worker-teardown ordering all become unnecessary and are removed. This
also closes two correctness holes the queue had: a deletion signal could be dropped
behind a pending serve job, and buffered image bytes had no global cap.
The in-memory dedup now remembers a checksum of the served bytes plus the persisted
version: identical serves skip the decode and the write, but the same bytes under a
newer entity version re-persist, so blur_hash_updated_at keeps pace with row updates
and the Jellyfin DTO's staleness gate keeps emitting the stored hash after scans.
Placeholder-triggered clears check the seen map, then the stored row, so coverless
entities cost one row read once per process instead of a probe and write per serve.
UpdateBlurHash is a plain UPDATE with no user filtering, so the request context is
used directly (a client abort is survived via context.WithoutCancel).
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