feat: Phase 4 hardening — retry/backoff, loop detection, sync status UI + tests

Phase 4.1 — Retry/backoff for failed permission pushes:
- _compute_next_retry(): capped exponential backoff with jitter (60s–1h)
- _record_push_result(): tracks push_attempts and next_retry_at per link
- replay_pending_pushes(): skips links still in backoff window, logs deferred count
- _load() migration: adds push_attempts/next_retry_at to existing records

Phase 4.2 — Loop detection (A→B→A routing cycle):
- set_peer_route_via(): returns 409 if target cell already routes peers through us
- apply_remote_permissions(): soft warning when accepting exit-relay that would cycle

Phase 4.3 — Sync staleness indicator in Cell Network UI:
- SyncBadge component: green (synced), amber (pending/failed), gray (never)
- Shows relativeTime of last sync + error message + next retry estimate
- Injected into CellPanel header alongside tunnel online/handshake status

Tests (54 new):
- TestCheckInviteConflicts: subnet overlap, domain conflict, exclude_cell (9 tests)
- TestPushInviteToRemote: success, 4xx, no endpoint, subprocess errors (7 tests)
- TestAcceptInviteNew: new cell, idempotent, healing dns/subnet changes (16 tests)
- TestAddConnectionMutualPairing: push-invite call, non-fatal failure (5 tests)
- TestPeerSyncAcceptInvite endpoint: happy path, field validation, error propagation (16 tests)
- Fixed 2 existing replay tests to clear backoff gate (simulates elapsed window)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

webui/src/pages/CellNetwork.jsx

@@ -54,6 +54,31 @@ function StatusDot({ online }) {
     : <span className="inline-block h-2 w-2 rounded-full bg-red-400 mr-1.5" title="Offline" />;
 }
 
+function SyncBadge({ conn }) {
+  const { last_push_status, last_push_at, last_push_error, pending_push, next_retry_at } = conn;
+  let color, label, tip;
+  if (last_push_status === 'never' || (!last_push_at && !pending_push)) {
+    color = 'bg-gray-300'; label = 'Sync pending';
+    tip = 'Permissions not yet synced to remote cell';
+  } else if (!pending_push && last_push_status === 'ok') {
+    color = 'bg-green-500'; label = `Synced${last_push_at ? ' ' + relativeTime(last_push_at) : ''}`;
+    tip = `Permissions last synced ${last_push_at ? relativeTime(last_push_at) : ''}`;
+  } else {
+    color = 'bg-amber-400'; label = 'Out of sync';
+    tip = last_push_error ? `Sync failed: ${last_push_error}` : 'Permissions pending sync';
+    if (next_retry_at) {
+      const retryIn = Math.max(0, Math.round((new Date(next_retry_at) - Date.now()) / 60000));
+      tip += ` — next retry in ~${retryIn}m`;
+    }
+  }
+  return (
+    <span className="inline-flex items-center gap-1 text-xs text-gray-500" title={tip}>
+      <span className={`inline-block h-2 w-2 rounded-full ${color}`} />
+      {label}
+    </span>
+  );
+}
+
 function Toast({ toasts }) {
   return (
     <div className="fixed bottom-4 right-4 z-50 space-y-2">
@@ -204,6 +229,7 @@ function CellPanel({ conn, onDisconnect, addToast }) {
                 {conn.last_handshake && (
                   <span>{relativeTime(conn.last_handshake)}</span>
                 )}
+                <SyncBadge conn={conn} />
               </div>
             </div>
           </div>