feat(discover): show why each item is suggested

Each suggestion now carries the followed artists (seeds) that surfaced it,
from DiscoverySeedContribution joined to the current watched roster (an
unfollowed seed drops out), highest-scoring seed first. /discover renders
"Similar to Arctic Monkeys, Coldplay +2" under each artist and album row —
both kinds key off the candidate's artistMbid, so an album inherits its
artist's seeds. web 187 tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/app/design.css

@@ -285,6 +285,7 @@ nav.contents .sep { flex: 1; }
 }
 .save-row { display: flex; align-items: center; gap: 14px; margin-top: 8px; }
 .edit-meta { display: flex; flex-direction: column; gap: 8px; width: 100%; }
+.rmeta.why { font-style: italic; color: var(--graphite); margin-top: 2px; }
 .sel-tools { display: inline-flex; align-items: center; gap: 10px; }
 .bulk-bar { display: flex; align-items: center; gap: 12px; flex-wrap: wrap; margin: 4px 0 12px; padding: 8px 12px; border: 1px solid var(--rule); background: var(--paper-2); }
 .album-card { position: relative; }