P2-05: forget command with retention policy

End-to-end forget plumbing — operator can create a forget schedule
with keep-* values, agent runs restic forget --keep-* … on the
schedule's cron (or via per-row Run-now), snapshot list shrinks,
UI updates.

* api.CommandRunPayload gains retention_policy json.RawMessage so
  the agent doesn't need a typed copy of the server-side struct.
* restic.ForgetPolicy mirrors restic's --keep-* flags. Empty()
  reports zero dimensions; restic wrapper RunForget refuses to
  run an empty policy (would delete every snapshot). Does NOT
  pass --prune — pruning lives behind a separate admin-only
  credential (P2-06); forget just rewrites the snapshot index.
* runner.RunForget mirrors RunBackup's envelope shape so the
  live log viewer works without special-casing. On success
  triggers reportSnapshots (forget shrinks the index, the host's
  snapshot count almost certainly changed).
* cmd/agent dispatcher handles MsgCommandRun with kind=forget,
  decodes RetentionPolicy from the wire, builds restic.ForgetPolicy.
* Server dispatchScheduleNow marshals the schedule's
  RetentionPolicy into the wire payload for kind=forget jobs.
  Refuses to dispatch a forget schedule with empty retention.
* validateSchedule rejects kind=forget without at least one keep-*
  dimension (new error code: missing_retention).
* UI schedule edit form gains a Kind dropdown (backup or forget;
  immutable on edit). Paths block toggles by kind via inline
  data-kind attributes. Form help-text explains the prune
  separation.

Other kinds (prune, check, unlock) deferred to P2-06..08; the
Kind dropdown only offers backup and forget today.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/server/http/schedule_push.go

@@ -187,6 +187,21 @@ func (s *Server) dispatchScheduleNow(ctx context.Context, hostID, scheduleID str
 		args = append(args, sched.Paths...)
 	}
 
+	// forget jobs need the retention policy on the wire — restic
+	// refuses to run without keep-* flags, and the agent doesn't
+	// hold a copy of the schedule (server is the source of truth).
+	var retentionJSON json.RawMessage
+	if sched.Kind == string(api.JobForget) {
+		if sched.RetentionPolicy == (store.RetentionPolicy{}) {
+			return "", errFmtf("schedule has no retention policy — refusing to forget (would delete every snapshot)")
+		}
+		b, err := json.Marshal(sched.RetentionPolicy)
+		if err != nil {
+			return "", errFmtf("marshal retention policy: %s", err)
+		}
+		retentionJSON = b
+	}
+
 	jobID := ulid.Make().String()
 	now := time.Now().UTC()
 	if err := s.deps.Store.CreateJob(ctx, store.Job{
@@ -202,9 +217,10 @@ func (s *Server) dispatchScheduleNow(ctx context.Context, hostID, scheduleID str
 	}
 
 	env, err := api.Marshal(api.MsgCommandRun, jobID, api.CommandRunPayload{
-		JobID: jobID,
-		Kind:  api.JobKind(sched.Kind),
-		Args:  args,
+		JobID:           jobID,
+		Kind:            api.JobKind(sched.Kind),
+		Args:            args,
+		RetentionPolicy: retentionJSON,
 	})
 	if err != nil {
 		return "", errFmtf("marshal command.run: %s", err)