feat(ui): add primary/backup endpoint fields to data connection form

src/ScadaLink.CentralUI/Components/Pages/Admin/DataConnectionForm.razor

@@ -55,10 +55,43 @@
                     </select>
                 </div>
                 <div class="mb-2">
-                    <label class="form-label small">Configuration (JSON)</label>
+                    <label class="form-label small">Primary Endpoint Configuration</label>
                     <input type="text" class="form-control form-control-sm" @bind="_formConfiguration"
                            placeholder='e.g. {"endpoint":"opc.tcp://..."}' />
                 </div>
+
+                @if (!_showBackup)
+                {
+                    <div class="mb-3">
+                        <button type="button" class="btn btn-outline-secondary btn-sm"
+                                @onclick="() => _showBackup = true">
+                            Add Backup Endpoint
+                        </button>
+                    </div>
+                }
+                else
+                {
+                    <div class="mb-3">
+                        <div class="d-flex justify-content-between align-items-center mb-1">
+                            <label class="form-label small mb-0">Backup Endpoint Configuration</label>
+                            <button type="button" class="btn btn-outline-danger btn-sm"
+                                    @onclick="RemoveBackup">
+                                Remove Backup
+                            </button>
+                        </div>
+                        <textarea class="form-control form-control-sm" rows="4"
+                                  @bind="_formBackupConfiguration"
+                                  placeholder='{"Host": "backup-host", "Port": 50101}' />
+                    </div>
+
+                    <div class="mb-3">
+                        <label class="form-label small">Failover Retry Count</label>
+                        <input type="number" class="form-control form-control-sm" style="max-width: 120px;"
+                               min="1" max="20"
+                               @bind="_formFailoverRetryCount" />
+                        <div class="form-text">Retries on active endpoint before switching to backup (default: 3)</div>
+                    </div>
+                }
                 @if (_formError != null)
                 {
                     <div class="text-danger small mt-2">@_formError</div>
@@ -83,6 +116,9 @@
     private string _formName = string.Empty;
     private string _formProtocol = string.Empty;
     private string? _formConfiguration;
+    private bool _showBackup;
+    private string? _formBackupConfiguration;
+    private int _formFailoverRetryCount = 3;
     private string? _formError;
 
     protected override async Task OnInitializedAsync()
@@ -101,6 +137,9 @@
                     _formName = _editingConnection.Name;
                     _formProtocol = _editingConnection.Protocol;
                     _formConfiguration = _editingConnection.PrimaryConfiguration;
+                    _formBackupConfiguration = _editingConnection.BackupConfiguration;
+                    _formFailoverRetryCount = _editingConnection.FailoverRetryCount;
+                    _showBackup = _editingConnection.BackupConfiguration != null;
                 }
             }
             catch (Exception ex)
@@ -125,13 +164,17 @@
                 _editingConnection.Name = _formName.Trim();
                 _editingConnection.Protocol = _formProtocol;
                 _editingConnection.PrimaryConfiguration = _formConfiguration?.Trim();
+                _editingConnection.BackupConfiguration = _showBackup ? _formBackupConfiguration?.Trim() : null;
+                _editingConnection.FailoverRetryCount = _showBackup ? _formFailoverRetryCount : 3;
                 await SiteRepository.UpdateDataConnectionAsync(_editingConnection);
             }
             else
             {
                 var conn = new DataConnection(_formName.Trim(), _formProtocol, _formSiteId)
                 {
-                    PrimaryConfiguration = _formConfiguration?.Trim()
+                    PrimaryConfiguration = _formConfiguration?.Trim(),
+                    BackupConfiguration = _showBackup ? _formBackupConfiguration?.Trim() : null,
+                    FailoverRetryCount = _showBackup ? _formFailoverRetryCount : 3
                 };
                 await SiteRepository.AddDataConnectionAsync(conn);
             }
@@ -144,6 +187,13 @@
         }
     }
 
+    private void RemoveBackup()
+    {
+        _showBackup = false;
+        _formBackupConfiguration = null;
+        _formFailoverRetryCount = 3;
+    }
+
     private void GoBack()
     {
         NavigationManager.NavigateTo("/admin/data-connections");

src/ScadaLink.CentralUI/Components/Pages/Admin/DataConnections.razor

@@ -32,7 +32,8 @@
                     <th>Name</th>
                     <th>Protocol</th>
                     <th>Site</th>
-                    <th>Configuration</th>
+                    <th>Primary Config</th>
+                    <th>Backup Config</th>
                     <th style="width: 160px;">Actions</th>
                 </tr>
             </thead>
@@ -40,7 +41,7 @@
                 @if (_connections.Count == 0)
                 {
                     <tr>
-                        <td colspan="6" class="text-muted text-center">No data connections configured.</td>
+                        <td colspan="7" class="text-muted text-center">No data connections configured.</td>
                     </tr>
                 }
                 @foreach (var conn in _connections)
@@ -51,6 +52,7 @@
                         <td><span class="badge bg-secondary">@conn.Protocol</span></td>
                         <td>@(_siteLookup.GetValueOrDefault(conn.SiteId)?.Name ?? $"Site {conn.SiteId}")</td>
                         <td class="text-muted small text-truncate" style="max-width: 300px;">@(conn.PrimaryConfiguration ?? "—")</td>
+                        <td class="text-muted small text-truncate" style="max-width: 300px;">@(conn.BackupConfiguration ?? "—")</td>
                         <td>
                             <button class="btn btn-outline-primary btn-sm py-0 px-1 me-1"
                                     @onclick='() => NavigationManager.NavigateTo($"/admin/data-connections/{conn.Id}/edit")'>Edit</button>