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/waf/observe_test.go

@@ -12,7 +12,7 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/requestlog"
 )
 
-// observe is the value of SWW_MODE for observe mode.
+// observe is the value of SWW_WAF_MODE for observe mode.
 const observe = "observe"
 
 func TestBanActionSaysWhetherTheBanRunsOut(t *testing.T) {
@@ -118,12 +118,12 @@ func TestObserveModeForwardsWhatEnforceModeRefuses(t *testing.T) {
 	t.Parallel()
 
 	const (
-		denied = "192.0.2.50" // in SWW_DENY_NETS
+		denied = "192.0.2.50" // in SWW_WAF_DENY_NETS
 		banned = otherClient  // under a ban read from bans.json
 	)
 
 	for _, tc := range []struct {
-		setting string // "" leaves SWW_MODE at its default
+		setting string // "" leaves SWW_WAF_MODE at its default
 		observe bool
 	}{
 		{"", false},