Search Deezer and MusicBrainz together; add preview clips

Search
- Albums and songs search Deezer and MusicBrainz at once and merge the
  results: duplicates keep the Deezer copy, best word matches first.
  Deezer's search misses albums it carries (Death Magnetic) and most game
  soundtracks; MusicBrainz finds them
- MusicBrainz secondary types are shown ("Soundtrack", "Live")
- Settings: Deezer + MusicBrainz (default), Deezer only, MusicBrainz only
- Search results show which service they came from

Previews
- Songs and albums get a play button (cards, rows, detail view, editor,
  and Deezer search results) for a 30-second Deezer clip; albums play
  their most popular track
- Clips are looked up fresh (Deezer URLs expire), cached 10 minutes, and
  streamed through Canon with Range support; hidden lists stay private
- Entries from MusicBrainz or added by hand are matched to Deezer on save
  so they can preview too
- "Play previews" toggle in Settings → Profile

Tests cover merging and title/artist matching; `-tags live` runs checks
against the real services.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/templates/partials/items.html

@@ -19,6 +19,9 @@
         {{- with .Note}}<span class="item-note">{{.}}</span>{{end}}
       </span>
     </button>
+    {{- if .Preview}}
+    <button type="button" class="play-btn" data-preview-src="/preview/{{.ID}}" aria-pressed="false" aria-label="Play a preview of {{.Title}}" title="Play 30-second preview">{{icon "play"}}{{icon "pause"}}</button>
+    {{- end}}
   </li>
   {{- end}}
 </ol>