Add alternative names and cover art for soundtracks

Alternative names (#32):
- PUT /api/v1/soundtracks/{uuid}/alternative-names replaces a soundtrack's
  alternative names (trimmed, deduplicated, without the soundtrack's own name).
- alternative_names is in the soundtrack and statistics responses.
- /find matches them too and shows which name matched.

Covers (#12), stored in COVERS_PATH and off when it is not set:
- GET, PUT (upload) and DELETE /api/v1/soundtracks/{uuid}/cover, and
  PUT .../cover/source to pick a SteamGridDB game or AniList anime by ID.
  Uploaded and picked covers are never replaced automatically.
- With STEAMGRIDDB_API_KEY, a background job searches SteamGridDB, then
  AniList, by name, alternative names and the name without bracketed parts,
  at startup, after a sync and after names change. Search results are ranked
  by matching words, so "Civilization IV" gets "Sid Meier's Civilization IV".
- cover_modified (Unix seconds) is in the soundtrack and statistics responses,
  so clients can cache covers.

The new columns are added to migration 000006, which is not released yet.

Closes #12, closes #32

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

cmd/web/search.templ

@@ -40,10 +40,13 @@ templ SearchForm() {
 	}
 }
 
-templ FoundSoundtracks(soundtracks []string) {
+templ FoundSoundtracks(soundtracks []SearchResult) {
 	for _, soundtrack := range soundtracks {
 		<div class="bg-green-100 p-4 shadow-md rounded-lg mt-6">
-			<p class="soundtrack-text">{ soundtrack }</p>
+			<p class="soundtrack-text">{ soundtrack.Name }</p>
+			if soundtrack.MatchedName != "" {
+				<p class="matched-name">Also known as { soundtrack.MatchedName }</p>
+			}
 		</div>
 	}
 }