feat: single Docker image with frontend served from backend

- Dockerfile: 3-stage build (frontend npm build → backend go build → alpine runtime)
- Frontend dist files served from disk at /app/frontend/dist
- server.go: catch-all GET handler serves static files with SPA fallback to index.html
- config.go: new server.frontend_dir option (env: SUB_STORE_SERVER_FRONTEND_DIR)
- API routes (/api/*), download routes, and /health take priority over static serving

internal/config/config.go

@@ -22,6 +22,7 @@ type ServerConfig struct {
 	ReadTimeout  time.Duration `mapstructure:"read_timeout"`
 	WriteTimeout time.Duration `mapstructure:"write_timeout"`
 	BodyLimit    int           `mapstructure:"body_limit"`
+	FrontendDir  string        `mapstructure:"frontend_dir"`
 }
 
 type DatabaseConfig struct {
@@ -62,6 +63,7 @@ func defaults() {
 	viper.SetDefault("server.read_timeout", 30*time.Second)
 	viper.SetDefault("server.write_timeout", 60*time.Second)
 	viper.SetDefault("server.body_limit", 4*1024*1024)
+	viper.SetDefault("server.frontend_dir", "./frontend/dist")
 	viper.SetDefault("database.path", "./data/sub-store.db")
 	viper.SetDefault("auth.admin_token", "")
 	viper.SetDefault("auth.download_token", "")