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_.
Every place a name appeared changed: the code, the viper keys, tests,
documents, examples, the README's settings table and the image files.

Model: deepseek-v4-pro

internal/discriminator/errorburst_test.go

@@ -15,7 +15,7 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/requestlog"
 )
 
-const errorBurstThreshold = "SWW_ERROR_BURST_THRESHOLD"
+const errorBurstThreshold = "SWW_WAF_ERROR_BURST_THRESHOLD"
 
 // refused is a request the tests here send, which discriminator refuses after a rule
 // file match.
@@ -48,7 +48,7 @@ func (r refused) send(s *sender, from string) logLine {
 }
 
 // startForErrorBurst is startWithClock with testRules, the metrics token and
-// SWW_ERROR_BURST_THRESHOLD at threshold, and the settings in env.
+// SWW_WAF_ERROR_BURST_THRESHOLD at threshold, and the settings in env.
 func startForErrorBurst(
 	t *testing.T, threshold string, env map[string]string,
 ) (*sender, *clock, *discriminator.Server) {
@@ -258,15 +258,15 @@ func TestErrorBurstCountsEachClientTheChecksApplyTo(t *testing.T) {
 	t.Parallel()
 
 	const (
-		allowed = "192.0.2.60" // in SWW_ALLOW_NETS
-		exempt  = "192.0.2.50" // in SWW_RATE_LIMIT_EXEMPT_NETS
+		allowed = "192.0.2.60" // in SWW_WAF_ALLOW_NETS
+		exempt  = "192.0.2.50" // in SWW_WAF_RATE_LIMIT_EXEMPT_NETS
 	)
 
 	s, _, _ := startForErrorBurst(t, "1", map[string]string{
 		allowNets: allowed, rateLimitExemptNets: exempt,
 	})
 
-	// A client in SWW_ALLOW_NETS is not refused, and so not counted.
+	// A client in SWW_WAF_ALLOW_NETS is not refused, and so not counted.
 	for range 3 {
 		s.request(allowed, blockedPath, http.StatusOK, requestlog.ActionForward)
 	}