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>

cmd/agent/main.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"context"
+	"encoding/json"
 	"errors"
 	"flag"
 	"fmt"
@@ -279,6 +280,34 @@ func (d *dispatcher) runJob(ctx context.Context, p api.CommandRunPayload, tx wsc
 			}
 			slog.Info("agent: init job complete", "job_id", p.JobID)
 		}()
+	case api.JobForget:
+		var policy restic.ForgetPolicy
+		if len(p.RetentionPolicy) > 0 {
+			var raw struct {
+				KeepLast    *int `json:"keep_last,omitempty"`
+				KeepHourly  *int `json:"keep_hourly,omitempty"`
+				KeepDaily   *int `json:"keep_daily,omitempty"`
+				KeepWeekly  *int `json:"keep_weekly,omitempty"`
+				KeepMonthly *int `json:"keep_monthly,omitempty"`
+				KeepYearly  *int `json:"keep_yearly,omitempty"`
+			}
+			if err := json.Unmarshal(p.RetentionPolicy, &raw); err != nil {
+				return fmt.Errorf("forget: decode retention_policy: %w", err)
+			}
+			policy = restic.ForgetPolicy{
+				KeepLast: raw.KeepLast, KeepHourly: raw.KeepHourly,
+				KeepDaily: raw.KeepDaily, KeepWeekly: raw.KeepWeekly,
+				KeepMonthly: raw.KeepMonthly, KeepYearly: raw.KeepYearly,
+			}
+		}
+		slog.Info("agent: accepting forget job", "job_id", p.JobID, "policy", p.RetentionPolicy)
+		go func() {
+			if err := r.RunForget(ctx, p.JobID, policy); err != nil {
+				slog.Warn("agent: forget job failed", "job_id", p.JobID, "err", err)
+				return
+			}
+			slog.Info("agent: forget job complete", "job_id", p.JobID)
+		}()
 	default:
 		return fmt.Errorf("kind %q not implemented yet (Phase 2 lands the rest)", p.Kind)
 	}