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/library.go

@@ -3,23 +3,30 @@ package backend
 import (
 	"context"
 	"errors"
+	"strings"
 	"time"
 
 	"music-server/internal/db/repository"
 
 	"github.com/google/uuid"
 	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgtype"
 )
 
 // Soundtrack is a soundtrack as returned by the API.
 type Soundtrack struct {
-	Uuid        uuid.UUID  `json:"uuid"`
-	Name        string     `json:"name"`
-	TimesPlayed int32      `json:"times_played"`
-	LastPlayed  *time.Time `json:"last_played,omitempty"`
-	Added       time.Time  `json:"added"`
+	Uuid             uuid.UUID  `json:"uuid"`
+	Name             string     `json:"name"`
+	AlternativeNames []string   `json:"alternative_names" doc:"Other names the soundtrack is known by"`
+	TimesPlayed      int32      `json:"times_played"`
+	LastPlayed       *time.Time `json:"last_played,omitempty"`
+	Added            time.Time  `json:"added"`
+	CoverModified    *int64     `json:"cover_modified,omitempty" doc:"When the cover last changed, in Unix seconds. Missing when there is no cover."`
 }
 
+// MaxNameLength is the longest alternative name that can be stored, in bytes.
+const MaxNameLength = 200
+
 // SoundtrackWithSongList is a soundtrack with all its songs.
 type SoundtrackWithSongList struct {
 	Soundtrack
@@ -48,14 +55,65 @@ func NewLibraryService(q repository.Querier) *LibraryService {
 
 func toSoundtrack(s repository.Soundtrack) Soundtrack {
 	return Soundtrack{
-		Uuid:        s.Uuid,
-		Name:        s.SoundtrackName,
-		TimesPlayed: s.TimesPlayed,
-		LastPlayed:  s.LastPlayed,
-		Added:       s.Added,
+		Uuid:             s.Uuid,
+		Name:             s.SoundtrackName,
+		AlternativeNames: nonNil(s.AlternativeNames),
+		TimesPlayed:      s.TimesPlayed,
+		LastPlayed:       s.LastPlayed,
+		Added:            s.Added,
+		CoverModified:    unixSeconds(s.CoverModified),
 	}
 }
 
+// nonNil returns names, or an empty list for nil, so the JSON is [] and never null.
+func nonNil(names []string) []string {
+	if names == nil {
+		return []string{}
+	}
+	return names
+}
+
+// unixSeconds returns t in Unix seconds, or nil when it is not set.
+func unixSeconds(t pgtype.Timestamptz) *int64 {
+	if !t.Valid {
+		return nil
+	}
+	seconds := t.Time.Unix()
+	return &seconds
+}
+
+// SetAlternativeNames replaces the alternative names of an active soundtrack. The names
+// are trimmed, and empty names, duplicates and names equal to the soundtrack's own name
+// (ignoring case) are dropped.
+func (l *LibraryService) SetAlternativeNames(ctx context.Context, id uuid.UUID, names []string) (Soundtrack, error) {
+	row, err := l.q.GetSoundtrackByUUID(ctx, id)
+	if err != nil {
+		return Soundtrack{}, notFound(err)
+	}
+	row, err = l.q.SetAlternativeNames(ctx, repository.SetAlternativeNamesParams{
+		Uuid: id, Names: cleanAlternativeNames(row.SoundtrackName, names),
+	})
+	if err != nil {
+		return Soundtrack{}, notFound(err)
+	}
+	return toSoundtrack(row), nil
+}
+
+func cleanAlternativeNames(name string, names []string) []string {
+	cleaned := []string{}
+	seen := map[string]bool{strings.ToLower(strings.TrimSpace(name)): true}
+	for _, n := range names {
+		n = strings.TrimSpace(n)
+		key := strings.ToLower(n)
+		if n == "" || seen[key] {
+			continue
+		}
+		seen[key] = true
+		cleaned = append(cleaned, n)
+	}
+	return cleaned
+}
+
 func notFound(err error) error {
 	if errors.Is(err, pgx.ErrNoRows) {
 		return ErrNotFound