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

Every setting of discriminator 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. Discriminator'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/discriminator/countries.go

@@ -1,6 +1,6 @@
 package discriminator
 
-// The heuristic countries. It reads SWW_EXCLUSIVELY_ALLOWED_COUNTRIES.
+// The heuristic countries. It reads SWW_WAF_EXCLUSIVELY_ALLOWED_COUNTRIES.
 
 import (
 	"context"
@@ -10,7 +10,7 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/requestlog"
 )
 
-// countries refuses a request when SWW_EXCLUSIVELY_ALLOWED_COUNTRIES is set and
+// countries refuses a request when SWW_WAF_EXCLUSIVELY_ALLOWED_COUNTRIES is set and
 // does not hold the client's country as it was looked up. A client without a
 // country, or whose country cannot be found, is refused. Its figures are the
 // country and the setting that refused it.
@@ -29,7 +29,7 @@ func (countries) check(_ context.Context, rq *request, _ time.Time) *verdict {
 	return &verdict{
 		action: requestlog.ActionCountryDenied,
 		figures: map[string]any{
-			"country": country, "setting": "SWW_EXCLUSIVELY_ALLOWED_COUNTRIES",
+			"country": country, "setting": "SWW_WAF_EXCLUSIVELY_ALLOWED_COUNTRIES",
 		},
 	}
 }