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.go

@@ -33,15 +33,33 @@ func NewSearchHandler(cache *backend.SearchCache) http.Handler {
 	})
 }
 
+// SearchResult is a soundtrack found by search. MatchedName is the alternative name that
+// matched, or empty when the soundtrack's own name matched.
+type SearchResult struct {
+	Name        string
+	MatchedName string
+}
+
 // search returns the soundtracks matching searchText: exact matches first, then matches
-// ignoring spaces and special characters, then matches with the letters in order.
-func search(searchText string, soundtrackData []backend.SoundtrackData) []string {
-	var results []string
+// ignoring spaces and special characters, then matches with the letters in order. Each
+// step tries the soundtrack's own name before its alternative names.
+func search(searchText string, soundtrackData []backend.SoundtrackData) []SearchResult {
+	var results []SearchResult
 	seen := make(map[string]bool)
-	add := func(match func(backend.SoundtrackData) bool) {
+	add := func(match func(backend.SearchName) bool) {
 		for _, data := range soundtrackData {
-			if !seen[data.Original] && match(data) {
-				results = append(results, data.Original)
+			if seen[data.Original] {
+				continue
+			}
+			matched, ok := "", match(data.SearchName)
+			for _, alternative := range data.Alternatives {
+				if ok {
+					break
+				}
+				matched, ok = alternative.Original, match(alternative)
+			}
+			if ok {
+				results = append(results, SearchResult{Name: data.Original, MatchedName: matched})
 				seen[data.Original] = true
 			}
 		}
@@ -50,8 +68,8 @@ func search(searchText string, soundtrackData []backend.SoundtrackData) []string
 	lowercase := strings.ToLower(searchText)
 	cleaned := backend.CleanSearchTerm(searchText)
 
-	add(func(d backend.SoundtrackData) bool { return strings.Contains(d.Lowercase, lowercase) })
-	add(func(d backend.SoundtrackData) bool { return strings.Contains(d.Cleaned, cleaned) })
-	add(func(d backend.SoundtrackData) bool { return backend.MatchesSubsequence(cleaned, d.Cleaned) })
+	add(func(n backend.SearchName) bool { return strings.Contains(n.Lowercase, lowercase) })
+	add(func(n backend.SearchName) bool { return strings.Contains(n.Cleaned, cleaned) })
+	add(func(n backend.SearchName) bool { return backend.MatchesSubsequence(cleaned, n.Cleaned) })
 	return results
 }