Unset TRUSTED_PROXIES trusts RFC 1918, empty trusts none (closes #129)

The backend's own TRUSTED_PROXIES now follows the ruling for every app:
unset, it trusts 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16, and no
longer loopback; a list replaces that; set and empty, it trusts no
proxy.

viper reads an empty environment variable as unset, so an empty value
got the default. config.New now reads this one setting with
os.LookupEnv first. viper's AllowEmptyEnv would apply to every setting,
so an empty PORT or DEBUG would stop the start instead of counting as
unset, as README.md says it does.

bin/entrypoint.sh still starts the backend with
TRUSTED_PROXIES=127.0.0.1/32.

Model: opus-5-5

internal/config/config.go

@@ -9,6 +9,7 @@ import (
 	"math"
 	"net/netip"
 	"net/url"
+	"os"
 	"strconv"
 	"strings"
 
@@ -21,13 +22,11 @@ import (
 )
 
 // defaultTrustedProxies lists the networks whose forwarded
-// headers are honoured by default: IPv4 and IPv6 loopback,
-// for a reverse proxy on the same host, and the RFC1918
-// ranges. The container image does not use it:
+// headers are honoured when TRUSTED_PROXIES is unset: the
+// RFC 1918 ranges. The container image does not use it:
 // bin/entrypoint.sh gives the server 127.0.0.1/32, since
 // nginx is its only client there.
-const defaultTrustedProxies = "127.0.0.1/32,::1/128," +
-	"10.0.0.0/8,172.16.0.0/12,192.168.0.0/16"
+const defaultTrustedProxies = "10.0.0.0/8,172.16.0.0/12,192.168.0.0/16"
 
 // Default limits on stored reports; README.md gives the reasons for
 // these values.
@@ -135,6 +134,14 @@ func New(
 			viper.GetString("DEBUG"), errNotBool)
 	}
 
+	// An empty TRUSTED_PROXIES trusts no proxy, but viper reads an
+	// empty environment variable as unset and would give the default,
+	// so the environment is read first.
+	trustedProxies, set := os.LookupEnv("TRUSTED_PROXIES")
+	if !set {
+		trustedProxies = viper.GetString("TRUSTED_PROXIES")
+	}
+
 	s := &Config{
 		BindAddress:        viper.GetString("BIND_ADDRESS"),
 		CORSAllowedOrigins: splitList(viper.GetString("CORS_ALLOWED_ORIGINS")),
@@ -146,7 +153,7 @@ func New(
 		Port:               port,
 		ReportsPerMinute:   viper.GetInt("REPORTS_PER_MINUTE"),
 		SentryDSN:          viper.GetString("SENTRY_DSN"),
-		TrustedProxies:     splitList(viper.GetString("TRUSTED_PROXIES")),
+		TrustedProxies:     splitList(trustedProxies),
 		log:                log,
 		params:             &params,
 	}