The worker's WARNING observability proved the '38 errors' were almost all
MusicMap returning 404 (artist has no map page) — a genuine not-found, not a
fetch failure. But iter_musicmap_similar_artist_events flattened every
RequestException to status_code 502, and the worker maps 400/404 -> not_found
/ everything-else -> error, so these inflated the error count.
Surface the real HTTP status from the exception's response (404 stays 404),
falling back to 502 only when there's no response (timeout/connection drop,
which is correctly still an error eligible for retry).
Regression tests: 404 -> 404 (not_found), timeout -> 502 (error), 500 stays
error, plus an end-to-end worker check that a 404 result marks 'not_found'
and stores nothing.
