Read every setting through viper (closes #211)

servewaf's and rproxy's settings are read through viper, bound to the same environment variable names as before, 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, as it did. 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 tests pass set into a viper instead of a lookup function.

Model: deepseek-v4-pro

go.mod

@@ -10,12 +10,14 @@ require (
 	github.com/maxmind/mmdbwriter v1.2.0
 	github.com/oschwald/maxminddb-golang/v2 v2.7.0
 	github.com/prometheus/client_golang v1.24.1
+	github.com/spf13/viper v1.21.0
 )
 
 require (
 	github.com/beorn7/perks v1.0.1 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/corazawaf/libinjection-go v0.3.3 // indirect
+	github.com/go-viper/mapstructure/v2 v2.4.0 // indirect
 	github.com/goccy/go-json v0.10.5 // indirect
 	github.com/goccy/go-yaml v1.19.2 // indirect
 	github.com/gotnospirit/makeplural v0.0.0-20180622080156-a5f48d94d976 // indirect
@@ -25,14 +27,22 @@ require (
 	github.com/kylelemons/godebug v1.1.0 // indirect
 	github.com/magefile/mage v1.17.0 // indirect
 	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
+	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
 	github.com/petar-dambovaliev/aho-corasick v0.0.0-20250424160509-463d218d4745 // indirect
 	github.com/prometheus/client_model v0.6.2 // indirect
 	github.com/prometheus/common v0.70.1 // indirect
 	github.com/prometheus/procfs v0.21.1 // indirect
+	github.com/sagikazarmark/locafero v0.11.0 // indirect
+	github.com/sourcegraph/conc v0.3.1-0.20240121214520-5f936abd7ae8 // indirect
+	github.com/spf13/afero v1.15.0 // indirect
+	github.com/spf13/cast v1.10.0 // indirect
+	github.com/spf13/pflag v1.0.10 // indirect
+	github.com/subosito/gotenv v1.6.0 // indirect
 	github.com/tidwall/gjson v1.18.0 // indirect
 	github.com/tidwall/match v1.1.1 // indirect
 	github.com/tidwall/pretty v1.2.1 // indirect
 	github.com/valllabh/ocsf-schema-golang v1.0.3 // indirect
+	go.yaml.in/yaml/v3 v3.0.5 // indirect
 	go4.org/netipx v0.0.0-20231129151722-fdeea329fbba // indirect
 	golang.org/x/net v0.58.0 // indirect
 	golang.org/x/sync v0.23.0 // indirect