Read every setting through viper and rename the settings (closes #211)

Every setting of waf and rproxy is read through viper, in place of the
os.LookupEnv function Main passed down. NewViper turns AutomaticEnv and
AllowEmptyEnv on, so a setting that is set but empty reads as set. A
setting may also be given as a file, the _FILE form, which rproxy's
settings now have too; rproxy still writes nothing to disk.

The settings are renamed: waf's take the prefix SWW_WAF_, rproxy's take
SWW_RPROXY_, and the shared SWW_COMMAND keeps SWW_. The Core Rule Set's
settings take SWW_WAF_CORE_RULE_SET_ rather than SWW_WAF_WAF_.

Model: deepseek-v4-pro (first version); opus-5-5 (rework)

internal/config/config.go

@@ -1,10 +1,9 @@
 // Package config reads sww's settings, and is the one place they are
 // read. Every setting is an environment variable. The settings of `sww
-// waf` start with SWW_, every one of them has a default, and most can be
-// given as a file instead. Those of `sww rproxy` start with SWRP_. Each
-// of them but SWRP_WAF_BACKEND has a default, and none can be given as a file,
-// since rproxy never touches disk. SWW_COMMAND chooses which of the two
-// the image runs.
+// waf` start with SWW_WAF_, every one of them has a default, and most can be
+// given as a file instead. Those of `sww rproxy` start with SWW_RPROXY_. Each
+// of them but SWW_RPROXY_WAF_BACKEND has a default, and each can be given as a file
+// too. SWW_COMMAND chooses which of the two the image runs.
 package config
 
 import (
@@ -17,6 +16,8 @@ import (
 	"os"
 	"strconv"
 	"strings"
+
+	"github.com/spf13/viper"
 )
 
 // off is the value that switches a timeout, a size limit or a rate limit
@@ -27,8 +28,8 @@ const (
 	kibibyte = 1 << 10
 	mebibyte = 1 << 20
 	gibibyte = 1 << 30
-	// defaultListenAddr is the default of SWW_LISTEN_ADDR and
-	// SWRP_LISTEN_ADDR.
+	// defaultListenAddr is the default of SWW_WAF_LISTEN_ADDR and
+	// SWW_RPROXY_LISTEN_ADDR.
 	defaultListenAddr = ":8080"
 )
 
@@ -48,8 +49,8 @@ var (
 // Command reads only SWW_COMMAND, the command the image runs and its
 // health check checks: rproxy when it is unset or empty, or waf. It cannot
 // be given as a file. Any other value is an error that names the setting.
-func Command(lookupEnv func(string) (string, bool)) (string, error) {
-	command, _ := lookupEnv("SWW_COMMAND")
+func Command(v *viper.Viper) (string, error) {
+	command, _ := readSetting(v, "SWW_COMMAND")
 
 	switch command {
 	case "", "rproxy":
@@ -62,11 +63,23 @@ func Command(lookupEnv func(string) (string, bool)) (string, error) {
 }
 
 // environment is where FromEnvironment reads the settings: it notes each
-// value for the log, and keeps the first error.
+// value for the log, and keeps the first error. It reads every setting
+// through v, the viper NewViper returns, and honours each setting's _FILE
+// form too.
 type environment struct {
-	lookupEnv func(string) (string, bool)
-	settings  []slog.Attr
-	err       error
+	v        *viper.Viper
+	settings []slog.Attr
+	err      error
+}
+
+// readSetting returns a setting's value and whether it is set, reading it
+// through v.
+func readSetting(v *viper.Viper, name string) (string, bool) {
+	if !v.IsSet(name) {
+		return "", false
+	}
+
+	return v.GetString(name), true
 }
 
 // value returns a setting's value, or its default when it is not set,
@@ -87,10 +100,10 @@ func (e *environment) value(name, defaultValue string) string {
 // names, less one newline at their end. It notes that file's path for the
 // log. Both variables set, or a file that cannot be read, is an error.
 func (e *environment) lookup(name string) (string, bool) {
-	value, set := e.lookupEnv(name)
+	value, set := readSetting(e.v, name)
 	fileName := name + "_FILE"
 
-	path, inFile := e.lookupEnv(fileName)
+	path, inFile := readSetting(e.v, fileName)
 	if !inFile {
 		return value, set
 	}