PR #853 added artist album-list caching to Deezer, but unlike the Spotify path it
had no equivalent of the truncated-fetch guard: Deezer paginates, and a transient/
malformed response on page 2+ (artist with >100 albums) broke the loop and cached
the PARTIAL list as the full discography — serving an incomplete album list from
cache until TTL.
Fix: track whether pagination finished cleanly. A malformed/empty-of-data response
mid-walk now clears a `complete` flag and the artist→album-LIST is cached only when
complete. Individual album entities still cache regardless (each is complete; we
just have fewer). A clean end (short page / empty page / reached limit) still caches
as before.
Tests: a page-1-ok / page-2-errors walk no longer caches (second call refetches
instead of serving a permanently-incomplete list); a clean two-page walk still
caches (happy path intact). 181 deezer/metadata tests green.
