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/coreruleset/mastodon_test.go

@@ -11,7 +11,7 @@ import (
 	"sneak.berlin/go/smallwebwaf/internal/coreruleset"
 )
 
-// mastodonThreshold is SWW_WAF_ANOMALY_THRESHOLD's default.
+// mastodonThreshold is SWW_WAF_CORE_RULE_SET_ANOMALY_THRESHOLD's default.
 const mastodonThreshold = 5
 
 // mastodonStatuses is where Mastodon's apps and web app post.