[management] Record reverse proxy usage for activity accounting (#7116)

People who only ever reach private services through the reverse proxy were
invisible to activity accounting. Active users are counted from user.LastLogin
or from the LastSeen of a peer they own, and neither column was written on the
proxy paths — so a person signing in via SSO to a proxied service, or a peer
serving one over the mesh, never showed up in the 24 hour numbers.
Both writes now happen where the proxy already authenticates:
- GenerateSessionToken stamps LastLogin after the session token is signed,
  the same column and the same way the dashboard and device login paths do.
- ValidateTunnelPeer stamps the calling peer's LastSeen, the column its owner
  activates through.
The policy lives in a new reverseproxy/activity manager rather than in the gRPC
service, matching the module layout the other reverse proxy domains use. It
skips what can never count — service users, embedded proxy peers and WASM
clients — and throttles peer writes to once an hour, well inside the window
accounting asks about and far above the proxy's five minute tunnel cache.
The peer write is a single indexed UPDATE that touches only
peer_status_last_seen. Connected and SessionStartedAt are left alone so the
session-ownership fencing MarkPeerConnectedIfNewerSession relies on is never
disturbed, and the timestamp comes from the database clock rather than the
caller, for the same reason the other status writers take it from there. The
caller's cutoff travels into the statement's WHERE, so concurrent requests for
one peer collapse into a single write instead of each acting on its own stale
read, and a peer that was never seen — NULL last seen, since Status is an
embedded pointer — still records its first activity.
Nothing outside the reverse proxy changes behaviour: the only addition
elsewhere is the RefreshPeerLastSeen store method the manager calls.

management/server/store/sql_store.go

@@ -599,6 +599,34 @@ func (s *SqlStore) ApproveAccountPeers(ctx context.Context, accountID string) (i
 	return int(result.RowsAffected), nil
 }
 
+// RefreshPeerLastSeen updates only peer_status_last_seen. Every other status
+// column is left untouched: peer_status_connected and
+// peer_status_session_started_at belong to the sync stream that owns the
+// session, and a blind write here would corrupt the fencing
+// MarkPeerConnectedIfNewerSession relies on.
+//
+// LastSeen comes from the database clock for the same reason it does there: a
+// Go-side timestamp is taken before the write and can land after a connect that
+// used CURRENT_TIMESTAMP, dragging the column backwards.
+//
+// staleBefore carries the caller's throttle into the same statement, so
+// concurrent requests for one peer collapse into a single write instead of
+// each racing on its own stale read. The column is nullable — Status is an
+// embedded pointer, so a peer stored without one leaves it NULL — and NULL
+// loses every comparison, hence the explicit branch for a peer never seen.
+func (s *SqlStore) RefreshPeerLastSeen(ctx context.Context, accountID, peerID string, staleBefore time.Time) (bool, error) {
+	result := s.db.WithContext(ctx).
+		Model(&nbpeer.Peer{}).
+		Where(accountAndIDQueryCondition, accountID, peerID).
+		Where("(peer_status_last_seen IS NULL OR peer_status_last_seen < ?)", staleBefore).
+		Update("peer_status_last_seen", gorm.Expr("CURRENT_TIMESTAMP"))
+	if result.Error != nil {
+		return false, status.Errorf(status.Internal, "refresh peer last seen: %v", result.Error)
+	}
+
+	return result.RowsAffected > 0, nil
+}
+
 // SaveUsers saves the given list of users to the database.
 func (s *SqlStore) SaveUsers(ctx context.Context, users []*types.User) error {
 	if len(users) == 0 {