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/ratelimits_test.go

@@ -10,7 +10,7 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/requestlog"
 )
 
-// minute is the window of SWW_RATE_LIMIT_PER_MINUTE, as a log line's
+// minute is the window of SWW_WAF_RATE_LIMIT_PER_MINUTE, as a log line's
 // limit_hit names it.
 const minute = "minute"
 
@@ -75,7 +75,7 @@ func TestRateLimitRefusesBeforeTheApp(t *testing.T) {
 func TestIPv6GroupPrefixSetsTheClientTheLimitsCount(t *testing.T) {
 	t.Parallel()
 
-	// With SWW_IPV6_GROUP_PREFIX at 48, the first two addresses, in two
+	// With SWW_WAF_IPV6_GROUP_PREFIX at 48, the first two addresses, in two
 	// /64s of one /48, are one client, and the second's request breaks the
 	// limit; the third, in the next /48, is another client.
 	const (
@@ -118,7 +118,7 @@ func TestIPv6GroupPrefixSetsTheClientTheLimitsCount(t *testing.T) {
 func TestRateLimitExemptPathsAreNeitherCountedNorRefused(t *testing.T) {
 	t.Parallel()
 
-	const denied = "192.0.2.50" // in SWW_DENY_NETS
+	const denied = "192.0.2.50" // in SWW_WAF_DENY_NETS
 
 	geojsURL, _ := startGeoJS(t)
 	s, _, server := startWithClock(t, geojsURL, map[string]string{
@@ -151,7 +151,7 @@ func TestRateLimitExemptPathsAreNeitherCountedNorRefused(t *testing.T) {
 	// /assets/, and breaks the limit.
 	s.request(client, "/assets", http.StatusForbidden, requestlog.ActionRateLimited)
 
-	// A ban, SWW_DENY_NETS and the country lists still refuse a path
+	// A ban, SWW_WAF_DENY_NETS and the country lists still refuse a path
 	// under a prefix.
 	s.request(client, "/assets/app.js", http.StatusForbidden, requestlog.ActionBanned)
 	s.request(denied, "/assets/app.js", http.StatusForbidden, requestlog.ActionDenied)