Add ESPNow preset send backend support

Implement ESPNow helper model, WebSocket forwarding, and /presets/send endpoint that chunks and broadcasts presets to devices.

src/models/espnow.py

@@ -0,0 +1,61 @@
+import aioespnow
+
+
+class ESPNow:
+    """
+    Singleton ESPNow helper:
+    - Manages a single AIOESPNow instance
+    - Adds a single broadcast-like peer
+    - Exposes async send(data) to send to that peer.
+    """
+
+    _instance = None
+
+    def __new__(cls, *args, **kwargs):
+        if cls._instance is None:
+            cls._instance = super().__new__(cls)
+        return cls._instance
+
+    def __init__(self):
+        if getattr(self, "_initialized", False):
+            return
+
+        # Initialize ESPNow once (no disk persistence)
+        self._esp = aioespnow.AIOESPNow()
+        self._esp.active(True)
+
+   
+        try:
+            self._esp.add_peer(b"\xff\xff\xff\xff\xff\xff")
+        except Exception:
+            # Ignore add_peer failures (e.g. duplicate)
+            pass
+
+        self._initialized = True
+
+   
+    async def send(self, data):
+        """
+        Async send to the broadcast peer.
+        - data: bytes or str (JSON)
+        """
+        if isinstance(data, str):
+            payload = data.encode()
+        else:
+            payload = data
+
+        # Debug: show what we're sending and its size
+        try:
+            preview = payload.decode('utf-8')
+        except Exception:
+            preview = str(payload)
+        if len(preview) > 200:
+            preview = preview[:200] + "...(truncated)"
+        print("ESPNow.send len=", len(payload), "payload=", preview)
+
+        try:
+            await self._esp.asend(b"\xff\xff\xff\xff\xff\xff", payload)
+        except Exception as e:
+            # Log send failures but don't crash the app
+            print("ESPNow.send error:", e)
+