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)

AGENTS.md

@@ -30,7 +30,7 @@ These are the rules for agents working on this repository.
     - Secrets belong in POST data. An app that puts a secret in a query string
       is fixed in the app.
     - Never propose redacting a query string, Gitea's `?token=` included.
-- The default of `SWRP_TRUSTED_PROXIES` is the RFC 1918 set.
+- The default of `SWW_RPROXY_TRUSTED_PROXIES` is the RFC 1918 set.
     - The set is:
         - `10.0.0.0/8`;
         - `172.16.0.0/12`;