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/ui_schedules.go

@@ -34,6 +34,9 @@ type scheduleEditPage struct {
 	IsNew         bool
 	ScheduleID    string
 	Error         string
+	// Kind is settable on create, immutable on edit. The form's
+	// kind picker is hidden when !IsNew.
+	Kind          string
 	// Form values — strings so partial input survives validation
 	// errors (e.g. operator typed "abc" into keep_last).
 	CronExpr      string
@@ -110,6 +113,7 @@ func (s *Server) handleUIScheduleNewGet(w stdhttp.ResponseWriter, r *stdhttp.Req
 	view.Page = scheduleEditPage{
 		Host:     *host,
 		IsNew:    true,
+		Kind:     string(api.JobBackup),
 		CronExpr: "0 3 * * *",
 		Enabled:  true,
 	}
@@ -147,6 +151,7 @@ func (s *Server) handleUIScheduleEditGet(w stdhttp.ResponseWriter, r *stdhttp.Re
 		Host:        *host,
 		IsNew:       false,
 		ScheduleID:  sched.ID,
+		Kind:        sched.Kind,
 		CronExpr:    sched.CronExpr,
 		PathsRaw:    strings.Join(sched.Paths, "\n"),
 		ExcludesRaw: strings.Join(sched.Excludes, "\n"),
@@ -202,6 +207,7 @@ func (s *Server) handleUIScheduleSave(w stdhttp.ResponseWriter, r *stdhttp.Reque
 		Host:          *host,
 		IsNew:         scheduleID == "",
 		ScheduleID:    scheduleID,
+		Kind:          strings.TrimSpace(r.PostForm.Get("kind")),
 		CronExpr:      strings.TrimSpace(r.PostForm.Get("cron_expr")),
 		PathsRaw:      r.PostForm.Get("paths"),
 		ExcludesRaw:   r.PostForm.Get("excludes"),
@@ -217,6 +223,16 @@ func (s *Server) handleUIScheduleSave(w stdhttp.ResponseWriter, r *stdhttp.Reque
 		Enabled:       r.PostForm.Get("enabled") == "on",
 		Manual:        r.PostForm.Get("manual") == "on",
 	}
+	// Kind is immutable on edit — use the existing schedule's kind
+	// regardless of what the form submitted.
+	if !page.IsNew {
+		if existing, err := s.deps.Store.GetSchedule(r.Context(), hostID, scheduleID); err == nil {
+			page.Kind = existing.Kind
+		}
+	}
+	if page.Kind == "" {
+		page.Kind = string(api.JobBackup)
+	}
 
 	// Convert the raw form values into store-shape data, surfacing
 	// the first parse error as a banner.
@@ -238,13 +254,16 @@ func (s *Server) handleUIScheduleSave(w stdhttp.ResponseWriter, r *stdhttp.Reque
 	}
 
 	// Validate against the same rules the JSON API uses. Manual
-	// schedules skip the cron-expr requirement; everything else
-	// applies the same.
+	// schedules skip the cron-expr requirement; forget schedules
+	// require a non-empty retention policy. Other validation
+	// (kind in allowed set, paths required for backup, hooks
+	// rejected on non-backup) lives in validateSchedule.
 	apiShape := scheduleAPI{
-		Kind:     api.JobBackup,
-		CronExpr: page.CronExpr,
-		Paths:    paths,
-		Manual:   page.Manual,
+		Kind:            api.JobKind(page.Kind),
+		CronExpr:        page.CronExpr,
+		Paths:           paths,
+		Manual:          page.Manual,
+		RetentionPolicy: retention,
 	}
 	if code, msg := validateSchedule(&apiShape); code != "" {
 		page.Error = uiErrorMessage(code, msg)
@@ -256,7 +275,7 @@ func (s *Server) handleUIScheduleSave(w stdhttp.ResponseWriter, r *stdhttp.Reque
 		row := store.Schedule{
 			ID:              ulid.Make().String(),
 			HostID:          hostID,
-			Kind:            string(api.JobBackup),
+			Kind:            page.Kind,
 			CronExpr:        page.CronExpr,
 			Paths:           paths,
 			Excludes:        excludes,
@@ -500,6 +519,8 @@ func uiErrorMessage(code, msg string) string {
 		return "Cron expression doesn't parse: " + msg
 	case "missing_paths":
 		return "At least one backup path is required (one per line)."
+	case "missing_retention":
+		return "Forget schedules need at least one Keep-* value, otherwise restic would delete every snapshot."
 	case "invalid_kind":
 		return "Unsupported schedule kind."
 	default: