Allow and deny lists per subnet; every option of a DHCP message in words

- A subnet is a deny-list subnet (the default: everyone not denied) or
  an allow-list subnet (only clients allowed or with a reservation);
  the deny list counts in both. Entries are MAC addresses, prefixes
  (aa:bb:cc:*) or client IDs. A refused client gets no answer (DENIED
  in the activity log, with an Allow button) or a NAK on renewal.
  steward access, the subnet's Access tab. Schema migration 7.
- The activity log keeps each message's header fields and options,
  decoded into words (dhcp4.Describe): the parameter request list by
  name, client IDs, lease times. ▸ on an Activity row, steward
  activity show N / --details, GET /events/{id}.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/dhcpd/server.go

@@ -15,6 +15,7 @@ import (
 	"sync/atomic"
 	"time"
 
+	"steward/internal/dhcp4"
 	"steward/internal/model"
 )
 
@@ -324,6 +325,18 @@ type Event struct {
 	IP        string    `json:"ip,omitempty"`
 	Hostname  string    `json:"hostname,omitempty"`
 	Detail    string    `json:"detail,omitempty"`
+	// ID numbers the events since stewardd started.
+	ID uint64 `json:"id"`
+	// Direction is "in" for a client's message, "out" for Steward's
+	// answer, "" for an event of Steward's own (CONFLICT, NO ADDRESS).
+	Direction string `json:"direction,omitempty"`
+	ClientID  string `json:"client_id,omitempty"`
+	// SubnetID is the subnet that refused a client (DENIED).
+	SubnetID int64 `json:"subnet_id,omitempty"`
+	// Fields and Options are the message, in words (GET /events/{id}, or
+	// ?details=1).
+	Fields  []dhcp4.Field `json:"fields,omitempty"`
+	Options []dhcp4.Field `json:"options,omitempty"`
 }
 
 type events struct {
@@ -331,11 +344,14 @@ type events struct {
 	max    int
 	list   []Event
 	counts map[string]uint64
+	lastID uint64
 }
 
 func (e *events) add(ev Event) {
 	e.mu.Lock()
 	defer e.mu.Unlock()
+	e.lastID++
+	ev.ID = e.lastID
 	e.list = append(e.list, ev)
 	if len(e.list) > e.max {
 		e.list = append([]Event(nil), e.list[len(e.list)-e.max:]...)
@@ -362,6 +378,18 @@ func (s *Server) Events(limit int, keep func(Event) bool) []Event {
 	return out
 }
 
+// Event returns one event, with its details.
+func (s *Server) Event(id uint64) (Event, bool) {
+	s.events.mu.Lock()
+	defer s.events.mu.Unlock()
+	for i := len(s.events.list) - 1; i >= 0; i-- {
+		if s.events.list[i].ID == id {
+			return s.events.list[i], true
+		}
+	}
+	return Event{}, false
+}
+
 // Counts returns how many messages of each type were seen and sent since
 // the start.
 func (s *Server) Counts() map[string]uint64 {