Multiple failed source attempts at one song each land in quarantine as
separate entries. Group them by the *intended* target (sidecar context
track_info isrc -> id -> uri, falling back to normalized artist|title for
legacy thin sidecars) — an exact relationship across siblings, since the
bad files' own tags differ but the target track is constant.
- core: quarantine_group_key() + find_quarantine_siblings() seams; list
entries now carry group_key.
- approve endpoint: remove_siblings flag auto-deletes the other attempts
once one is accepted (captured BEFORE approve restores the file out of
quarantine, or the id lookup would resolve nothing). Scoped to the
quarantine manager; download-modal chooser + version-mismatch fallback
pass no flag and are unaffected.
- UI: multi-member groups render as a collapsible parent row (album art +
'N alternatives'); singletons unchanged. Toast reports removed count.
- 11 tests incl. ordering regression for capture-before-approve.
46be97b195