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

go.mod

@@ -17,6 +17,7 @@ require (
 	github.com/mdlayher/genetlink v1.3.2 // indirect
 	github.com/mdlayher/netlink v1.7.3-0.20250113171957-fbb4dce95f42 // indirect
 	github.com/mdlayher/socket v0.5.1 // indirect
+	github.com/oschwald/maxminddb-golang v1.13.1 // indirect
 	github.com/vishvananda/netns v0.0.5 // indirect
 	golang.org/x/net v0.58.0 // indirect
 	golang.org/x/sync v0.23.0 // indirect