extracted cover art handling from library package

backend/coverart/handler.go

@@ -0,0 +1,51 @@
+package coverart
+
+import (
+	"fmt"
+	"net/http"
+	"path/filepath"
+)
+
+// Handler serves cover art images via HTTP.
+type Handler struct {
+	coversDir string
+}
+
+// NewHandler creates an HTTP handler that serves cover art from the
+// user data directory.
+func NewHandler() (*Handler, error) {
+	dir, err := CoversDir()
+	if err != nil {
+		return nil, fmt.Errorf(
+			"could not resolve covers directory: %w", err,
+		)
+	}
+
+	return &Handler{coversDir: dir}, nil
+}
+
+// ServeHTTP handles requests for cover art images.
+func (h *Handler) ServeHTTP(
+	w http.ResponseWriter,
+	r *http.Request,
+) {
+	// Extract filename from path like "/covers/abc123.jpg".
+	filename := filepath.Base(r.URL.Path)
+
+	// Prevent directory traversal.
+	if filename == "." || filename == ".." {
+		http.NotFound(w, r)
+
+		return
+	}
+
+	// Filenames are content-hashed (SHA-256), so they are immutable.
+	// Set aggressive cache headers to avoid redundant re-fetches.
+	w.Header().Set(
+		"Cache-Control",
+		"public, max-age=31536000, immutable",
+	)
+
+	filePath := filepath.Join(h.coversDir, filename)
+	http.ServeFile(w, r, filePath)
+}