Add connection history per peer with country and network lookup

- The stats sampler records sessions per peer: start, end, address and
  traffic. A session ends when the peer goes quiet or is disabled; a new
  one starts when the device changes networks. Stored in stats.json and
  kept as long as the daily traffic history (max 1000 per peer).
- Country and network operator come from the free DB-IP Lite databases
  (CC BY 4.0), downloaded monthly and looked up locally, so peer
  addresses never leave the server. Settings → Data retention can switch
  this off, which deletes the databases.
- API: GET /peers/{id}/sessions; peer stats include the current location;
  settings include the database status.
- Web UI and iOS app: connection history card, location line, country
  code in the peer list (web), switch in data retention.

Claude-Session: https://claude.ai/code/session_01RAnLbyQZ5ZTA7KqwXP98nw

config.go

@@ -32,9 +32,14 @@ type Config struct {
 // StatsConfig sets how long traffic history is kept in stats.json.
 type StatsConfig struct {
 	HourlyHours int `json:"hourlyHours"` // hourly buckets, for the 24 h charts
-	DailyDays   int `json:"dailyDays"`   // daily buckets, for the 7/30/90 day charts
+	DailyDays   int `json:"dailyDays"`   // daily buckets and connection history
+	// GeoIP looks up country and network of peer addresses in the DB-IP Lite
+	// databases, downloaded monthly. Default on.
+	GeoIP *bool `json:"geoip,omitempty"`
 }
 
+func (c StatsConfig) geoEnabled() bool { return c.GeoIP == nil || *c.GeoIP }
+
 // Limits for the retention settings.
 const (
 	minLogSizeMB, maxLogSizeMB   = 1, 1000