missions: retry failed phases + auto-purge on re-launch

Every re-attempted phase now starts with a clean slate:

  - phase_runner::launch_phase DELETEs prior status IN ('failed',
    'cancelled') topology_runs for the phase before enqueuing the
    new ones. Completed runs are kept for audit; only the failure
    noise from earlier attempts goes.
  - POST /api/missions/{id}/phases/{phase_id}/retry — resets a
    failed/cancelled phase to 'pending' (auth-scoped to the calling
    workspace + guarded on mission.status='running'). phase_runner
    picks it up on the next 10s tick.
  - MissionCanvas phase card grows a coral 'Retry' button, visible
    only when phase.status='failed' and mission.status='running'.
    Click → resets + refreshes; the prior failed run rows disappear
    from the card as soon as phase_runner enqueues the new attempt.

Design: auto-purge in phase_runner rather than a separate 'clear
failed runs' endpoint. Users don't have to manually clean up before
retrying; the runner does it as part of the natural work of firing
a fresh attempt.

Verified: cargo check + tsc + eslint --quiet all green.

frontend/src/components/dashboard/MissionCanvas.tsx

@@ -16,6 +16,7 @@ import {
   getMission,
   listMissionRuns,
   refineMission,
+  retryMissionPhase,
   setMissionDescription,
   setMissionStatus,
   triggerBenchmark,
@@ -759,6 +760,37 @@ export function MissionCanvas({
                   })()}
                   {mission.status === "running" || mission.status === "completed" ? (
                     <div style={{ display: "flex", gap: 6, marginTop: 6 }}>
+                      {p.status === "failed" && mission.status === "running" && (
+                        <button
+                          type="button"
+                          onClick={async () => {
+                            if (phaseBusy === `retry:${p.id}`) return;
+                            setPhaseBusy(`retry:${p.id}`);
+                            setError(null);
+                            try {
+                              await retryMissionPhase(mission.id, p.id);
+                              await load();
+                            } catch (e) {
+                              setError(
+                                e instanceof Error ? e.message : "retry failed",
+                              );
+                            } finally {
+                              setPhaseBusy(null);
+                            }
+                          }}
+                          disabled={phaseBusy === `retry:${p.id}`}
+                          title="Reset this phase to pending; prior failed runs are purged. phase_runner picks it up in ≤10s."
+                          style={{
+                            ...secondaryBtn,
+                            borderColor: "rgba(255,138,122,.45)",
+                            color: "#ff8a7a",
+                            background: "rgba(255,138,122,.08)",
+                            opacity: phaseBusy === `retry:${p.id}` ? 0.5 : 1,
+                          }}
+                        >
+                          {phaseBusy === `retry:${p.id}` ? "Retrying…" : "Retry"}
+                        </button>
+                      )}
                       {p.kind === "security_scan" && (
                         <button
                           type="button"