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/main.py

@@ -459,6 +459,7 @@ def _hydrate_gate_store_from_chain(events: list[dict]) -> int:
     from services.mesh.mesh_hashchain import gate_store
 
     count = 0
+    gate_ids_updated: set[str] = set()
     for evt in events:
         if evt.get("event_type") != "gate_message":
             continue
@@ -471,6 +472,13 @@ def _hydrate_gate_store_from_chain(events: list[dict]) -> int:
             # don't corrupt the chain event's payload hash.
             gate_store.append(gate_id, copy.deepcopy(evt))
             count += 1
+            gate_ids_updated.add(gate_id)
+        except Exception:
+            pass
+    # Notify SSE clients so frontends refresh immediately.
+    for gid in gate_ids_updated:
+        try:
+            _broadcast_gate_events(gid, [{"hydrated": True}])
         except Exception:
             pass
     return count
@@ -920,7 +928,12 @@ _gate_sse_lock = threading.Lock()
 
 
 def _broadcast_gate_events(gate_id: str, events: list[dict]) -> None:
-    """Notify all connected SSE clients about new gate events (non-blocking)."""
+    """Notify all connected SSE clients about new gate events (non-blocking).
+
+    Called from background daemon threads (push/pull loops) AND the FastAPI
+    event-loop thread.  asyncio.Queue.put_nowait() is NOT thread-safe, so
+    background callers schedule via loop.call_soon_threadsafe().
+    """
     if not events:
         return
     payload = json_mod.dumps(
@@ -928,11 +941,32 @@ def _broadcast_gate_events(gate_id: str, events: list[dict]) -> None:
         separators=(",", ":"),
         ensure_ascii=False,
     )
+    # Detect whether we're already on the event-loop thread.
+    try:
+        asyncio.get_running_loop()
+        _in_loop = True
+    except RuntimeError:
+        _in_loop = False
+
+    _loop: asyncio.AbstractEventLoop | None = None
+    if not _in_loop:
+        try:
+            _loop = asyncio.get_event_loop()
+            if not _loop.is_running():
+                _loop = None
+        except RuntimeError:
+            _loop = None
+
     with _gate_sse_lock:
         dead: list[asyncio.Queue] = []
         for q in _gate_sse_clients:
             try:
-                q.put_nowait(payload)
+                if _in_loop:
+                    q.put_nowait(payload)
+                elif _loop is not None:
+                    _loop.call_soon_threadsafe(q.put_nowait, payload)
+                else:
+                    q.put_nowait(payload)  # best-effort fallback
             except (asyncio.QueueFull, Exception):
                 dead.append(q)
         for q in dead:

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,
 }