fix: thread-safe SSE broadcast + node enabled by default

- SSE broadcast now uses loop.call_soon_threadsafe() when called from
  background threads (gate pull/push loops), fixing silent notification
  failures for peer-synced messages
- Chain hydration path now broadcasts SSE so gate messages arriving via
  public chain sync trigger frontend refresh
- Node participation defaults to enabled so fresh installs automatically
  join the mesh network (push + pull)

backend/services/node_settings.py

@@ -10,7 +10,7 @@ _cache: dict | None = None
 _cache_ts: float = 0.0
 _CACHE_TTL = 5.0
 _DEFAULTS = {
-    "enabled": False,
+    "enabled": True,
 }