Add host network info display and custom subnet scan to Receivers tab

Shows the server's network interfaces (IP, subnet) in the admin Receivers
tab so users know which networks are being scanned. Adds an optional subnet
input field to scan arbitrary networks (e.g. VLANs the server can route to
but isn't directly attached to), with validation and a /20 size cap.

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

server.py

@@ -302,6 +302,25 @@ async def connect_to_receiver(host: str, port: int = 30003):
             await asyncio.sleep(reconnect_delay)
 
 
+async def _scan_subnet(network, port):
+    """Scan a single subnet for hosts with an open port. Returns list of IPs."""
+    found = []
+    print(f"Scanning {network} ({network.num_addresses} hosts) on port {port}...")
+    host_ips = [str(host) for host in network.hosts()]
+
+    async def check_ip(test_ip):
+        if await test_port(test_ip, port):
+            return test_ip
+        return None
+
+    results = await asyncio.gather(*[check_ip(h) for h in host_ips])
+    for result in results:
+        if result:
+            print(f"Found receiver at {result}:{port}")
+            found.append(result)
+    return found
+
+
 async def scan_for_receivers():
     """Scan local network for ADS-B receivers on configured port"""
     port = config['receiver_port']
@@ -317,20 +336,7 @@ async def scan_for_receivers():
                 if ip and netmask and not ip.startswith('127.'):
                     try:
                         network = ipaddress.IPv4Network(f"{ip}/{netmask}", strict=False)
-                        print(f"Scanning {network} ({network.num_addresses} hosts)...")
-
-                        host_ips = [str(host) for host in network.hosts()]
-
-                        async def check_ip(test_ip):
-                            if await test_port(test_ip, port):
-                                return test_ip
-                            return None
-
-                        results = await asyncio.gather(*[check_ip(h) for h in host_ips])
-                        for result in results:
-                            if result:
-                                print(f"Found receiver at {result}:{port}")
-                                found.append(result)
+                        found.extend(await _scan_subnet(network, port))
                     except ValueError as e:
                         print(f"Invalid network {ip}/{netmask}: {e}")
 
@@ -856,10 +862,58 @@ async def handle_admin_password(request):
     return web.json_response({"ok": True})
 
 
+@require_auth
+async def handle_admin_network_info(request):
+    """GET /api/admin/network-info - Return the server's network interfaces."""
+    interfaces = []
+    for iface in netifaces.interfaces():
+        addrs = netifaces.ifaddresses(iface)
+        if netifaces.AF_INET in addrs:
+            for addr in addrs[netifaces.AF_INET]:
+                ip = addr.get('addr')
+                netmask = addr.get('netmask')
+                if ip and netmask and not ip.startswith('127.'):
+                    try:
+                        network = ipaddress.IPv4Network(f"{ip}/{netmask}", strict=False)
+                        interfaces.append({
+                            "name": iface,
+                            "ip": ip,
+                            "netmask": netmask,
+                            "network": str(network)
+                        })
+                    except ValueError:
+                        pass
+    return web.json_response({"interfaces": interfaces})
+
+
 @require_auth
 async def handle_admin_scan_receivers(request):
     """POST /api/admin/scan-receivers - Trigger a receiver scan."""
-    found = await scan_for_receivers()
+    # Check for optional subnet in request body
+    subnet = None
+    try:
+        body = await request.json()
+        subnet = body.get("subnet")
+    except:
+        pass  # No body or invalid JSON — fall back to full scan
+
+    if subnet:
+        # Validate and scan a specific subnet
+        try:
+            network = ipaddress.IPv4Network(subnet, strict=False)
+        except (ValueError, TypeError):
+            return web.json_response(
+                {"error": f"Invalid subnet: {subnet}"}, status=400)
+
+        if network.prefixlen < 20:
+            return web.json_response(
+                {"error": "Subnet too large (max /20 = 4096 hosts)"}, status=400)
+
+        port = config['receiver_port']
+        found = await _scan_subnet(network, port)
+    else:
+        found = await scan_for_receivers()
+
     return web.json_response({"receivers": found})
 
 
@@ -1046,6 +1100,7 @@ async def start_http_server():
     app.router.add_put('/api/admin/themes/{id}', handle_admin_theme_rename)
     app.router.add_post('/api/admin/themes/{id}/{direction}', handle_admin_theme_upload)
     app.router.add_post('/api/admin/password', handle_admin_password)
+    app.router.add_get('/api/admin/network-info', handle_admin_network_info)
     app.router.add_post('/api/admin/scan-receivers', handle_admin_scan_receivers)
     app.router.add_post('/api/admin/restart-receivers', handle_admin_restart_receivers)
     app.router.add_get('/api/admin/sprites', handle_admin_sprites)