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>

internal/backend/search_cache.go

@@ -6,23 +6,40 @@ import (
 	"sync"
 )
 
-// SoundtrackData holds precomputed search data for a soundtrack
-type SoundtrackData struct {
+// SearchSoundtrack is a soundtrack's name and alternative names, as loaded for search.
+type SearchSoundtrack struct {
+	Name             string
+	AlternativeNames []string
+}
+
+// SearchName holds a name prepared for matching.
+type SearchName struct {
 	Original  string
 	Lowercase string
 	Cleaned   string
 }
 
-// SearchCache holds precomputed search data for all soundtrack names.
+func newSearchName(name string) SearchName {
+	return SearchName{Original: name, Lowercase: strings.ToLower(name), Cleaned: CleanSearchTerm(name)}
+}
+
+// SoundtrackData holds precomputed search data for a soundtrack: its name and its
+// alternative names.
+type SoundtrackData struct {
+	SearchName
+	Alternatives []SearchName
+}
+
+// SearchCache holds precomputed search data for all soundtracks.
 type SearchCache struct {
-	load func(context.Context) ([]string, error)
+	load func(context.Context) ([]SearchSoundtrack, error)
 
 	mu    sync.Mutex
 	cache []SoundtrackData
 }
 
-// NewSearchCache creates a SearchCache that gets the soundtrack names from load.
-func NewSearchCache(load func(context.Context) ([]string, error)) *SearchCache {
+// NewSearchCache creates a SearchCache that gets the soundtracks from load.
+func NewSearchCache(load func(context.Context) ([]SearchSoundtrack, error)) *SearchCache {
 	return &SearchCache{load: load}
 }
 
@@ -52,10 +69,9 @@ func (s *SearchCache) reload(ctx context.Context) error {
 	}
 	cache := make([]SoundtrackData, len(soundtracks))
 	for i, soundtrack := range soundtracks {
-		cache[i] = SoundtrackData{
-			Original:  soundtrack,
-			Lowercase: strings.ToLower(soundtrack),
-			Cleaned:   CleanSearchTerm(soundtrack),
+		cache[i] = SoundtrackData{SearchName: newSearchName(soundtrack.Name)}
+		for _, name := range soundtrack.AlternativeNames {
+			cache[i].Alternatives = append(cache[i].Alternatives, newSearchName(name))
 		}
 	}
 	s.cache = cache