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/config/config.go

@@ -31,6 +31,14 @@ type Config struct {
 	CharactersPath string
 	SoundCheckPath string
 	FrontendPath   string
+	// CoversPath is the folder for soundtrack covers. Empty turns covers off.
+	CoversPath string
+
+	// SteamGridDBAPIKey turns on the background search for covers.
+	SteamGridDBAPIKey string
+	// SteamGridDBURL and AniListURL replace the real cover APIs; only tests set them.
+	SteamGridDBURL string
+	AniListURL     string
 
 	LogLevel string
 	LogJSON  bool
@@ -65,6 +73,8 @@ func load(getenv func(string) string) (Config, error) {
 		CharactersPath:     getenv("CHARACTERS_PATH"),
 		SoundCheckPath:     withDefault(getenv("SOUND_CHECK_PATH"), "songs"),
 		FrontendPath:       withDefault(getenv("FRONTEND_PATH"), "/frontend"),
+		CoversPath:         getenv("COVERS_PATH"),
+		SteamGridDBAPIKey:  getenv("STEAMGRIDDB_API_KEY"),
 		LogLevel:           withDefault(getenv("LOG_LEVEL"), "info"),
 		LogJSON:            getenv("LOG_JSON") == "true",
 		APIPassword:        getenv("API_PASSWORD"),