Allow CORS origins from CORS_ALLOWED_ORIGINS

Echo compares allowed origins literally, so "https://*" and "http://*" never
matched a real origin and no CORS headers were sent. The allowed origins now
come from CORS_ALLOWED_ORIGINS (comma-separated, checked at startup). When it
is empty, only same-origin requests work, which is enough for the bundled
frontend.

Credentials are no longer allowed since the API uses Bearer tokens. Browsers
may now send Range and X-Queue-Id and read X-Queue-Id and Content-Range.

internal/config/config.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"net"
+	"net/url"
 	"os"
 	"strconv"
 	"strings"
@@ -38,6 +39,10 @@ type Config struct {
 	// TrustedProxies are CIDR ranges whose X-Forwarded-For header is trusted when resolving client IPs.
 	TrustedProxies []*net.IPNet
 
+	// CORSAllowedOrigins are the browser origins, like https://music.example.com, that may call the API
+	// from another site. Empty means only same-origin requests, which is enough for the bundled frontend.
+	CORSAllowedOrigins []string
+
 	ReleasesURL string
 }
 
@@ -91,6 +96,11 @@ func load(getenv func(string) string) (Config, error) {
 		errs = append(errs, fmt.Errorf("TRUSTED_PROXIES: %w", err))
 	}
 
+	cfg.CORSAllowedOrigins, err = parseOrigins(getenv("CORS_ALLOWED_ORIGINS"))
+	if err != nil {
+		errs = append(errs, fmt.Errorf("CORS_ALLOWED_ORIGINS: %w", err))
+	}
+
 	return cfg, errors.Join(errs...)
 }
 
@@ -110,6 +120,27 @@ func parseCIDRs(value string) ([]*net.IPNet, error) {
 	return nets, nil
 }
 
+// parseOrigins parses a comma-separated list of origins (scheme://host[:port], nothing else).
+func parseOrigins(value string) ([]string, error) {
+	var origins []string
+	for part := range strings.SplitSeq(value, ",") {
+		part = strings.TrimSpace(part)
+		if part == "" {
+			continue
+		}
+		u, err := url.Parse(part)
+		if err != nil {
+			return nil, err
+		}
+		if (u.Scheme != "http" && u.Scheme != "https") || u.Host == "" || u.Path != "" || u.RawQuery != "" ||
+			u.Fragment != "" || u.User != nil || strings.Contains(u.Host, "*") {
+			return nil, fmt.Errorf("%q is not an origin like https://music.example.com", part)
+		}
+		origins = append(origins, u.Scheme+"://"+u.Host)
+	}
+	return origins, nil
+}
+
 func withDefault(value, fallback string) string {
 	if value == "" {
 		return fallback