p1.1: dynamic self-IP deny (DDNS/trace, keep previous, atomic swap)

Discover WAN via ddns:<host> then trace:<url> before listen. Persist to
self_ip.json. Previous addresses stay denied for keep_previous. Failure
keeps the last known set. /healthz reports age and source.

internal/config/config.go

@@ -0,0 +1,51 @@
+package config
+
+import (
+	"os"
+	"time"
+
+	"github.com/pelletier/go-toml/v2"
+)
+
+// File is the on-disk /etc/sift/config.toml.
+type File struct {
+	Listen       string   `toml:"listen"`
+	UserAgent    string   `toml:"user_agent"`
+	DenyPrefixes []string `toml:"deny_prefixes"`
+	SelfIP       SelfIP   `toml:"self_ip"`
+}
+
+// SelfIP is the [self_ip] block.
+type SelfIP struct {
+	Sources      []string `toml:"sources"`
+	Interval     string   `toml:"interval"`
+	KeepPrevious string   `toml:"keep_previous"`
+	Path         string   `toml:"path"`
+}
+
+// Load reads path. A missing file returns a zero File and a nil error.
+func Load(path string) (File, error) {
+	var f File
+	b, err := os.ReadFile(path)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return f, nil
+		}
+		return f, err
+	}
+	if err := toml.Unmarshal(b, &f); err != nil {
+		return File{}, err
+	}
+	return f, nil
+}
+
+func ParseDuration(s string, def time.Duration) time.Duration {
+	if s == "" {
+		return def
+	}
+	d, err := time.ParseDuration(s)
+	if err != nil {
+		return def
+	}
+	return d
+}