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/coreruleset/nextcloud_test.go

@@ -13,7 +13,7 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/coreruleset"
 )
 
-// nextcloudThreshold is SWW_WAF_ANOMALY_THRESHOLD's default.
+// nextcloudThreshold is SWW_WAF_CORE_RULE_SET_ANOMALY_THRESHOLD's default.
 const nextcloudThreshold = 5
 
 // nextcloudSignIn is the sign-in page.
@@ -51,7 +51,7 @@ const (
 // nextcloudOther is a JSON field no exclusion names.
 const nextcloudOther = `{"other":"python3"}`
 
-// nextcloud is Nextcloud's name in SWW_APPS.
+// nextcloud is Nextcloud's name in SWW_WAF_APPS.
 const nextcloud = "nextcloud"
 
 func TestNextcloudFixesApplyOnlyToAHostnameThatServesNextcloud(
@@ -528,7 +528,7 @@ func TestNextcloudPasswordFieldsPass(t *testing.T) {
 func TestNextcloudUserPasswordInValuePasses(t *testing.T) {
 	t.Parallel()
 
-	const threshold = 5 // SWW_WAF_ANOMALY_THRESHOLD's default
+	const threshold = 5 // SWW_WAF_CORE_RULE_SET_ANOMALY_THRESHOLD's default
 
 	crs := readingBodies(t)
 	target := nextcloudUsers + "/alice"