feat(api): SHA-256 and Chromaprint fingerprints for the library (M5.5)

A background Fingerprinter runs after every scan and at startup, one file
at a time, filling media_files.sha256 and track_fingerprints (fpcalc raw,
first 120 s). Without fpcalc only SHA-256 is computed and files stay
queued for Chromaprint. Changing a file's content resets its prints.
Similarity() compares raw prints with a ±80-point alignment search; it is
the basis for duplicate detection in the upload queue.

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

services/api/internal/platform/config/config.go

@@ -39,6 +39,8 @@ type API struct {
 	StorageAgentURL string `env:"STORAGE_AGENT_URL" envDefault:"http://storage-agent:8090"`
 	InternalToken   string `env:"INTERNAL_TOKEN"`
 	FFprobePath     string `env:"FFPROBE_PATH" envDefault:"ffprobe"`
+	// FPCalcPath — fpcalc (Chromaprint) для акустических отпечатков; нет бинарника — считается только SHA-256.
+	FPCalcPath string `env:"FPCALC_PATH" envDefault:"fpcalc"`
 
 	// SigningKey — ключ подписи ссылок на обложки и стрим (base64, ≥ 16 байт). Пусто — случайный до перезапуска.
 	SigningKey string `env:"STREAM_SIGNING_KEY"`