feat: add backup/restore of app configurations

Add export and import functionality for app configurations:

- Export single app or all apps as versioned JSON backup bundle
- Import from backup file with name-conflict detection (skip duplicates)
- Fresh SSH keys and webhook secrets generated on import
- Preserves env vars, labels, volumes, and port mappings
- Web UI: export button on app detail, backup/restore page on dashboard
- REST API: GET /api/v1/apps/{id}/export, GET /api/v1/backup/export,
  POST /api/v1/backup/import
- Comprehensive test coverage for service and handler layers

templates/dashboard.html

@@ -11,12 +11,20 @@
 
     <div class="section-header">
         <h1 class="text-2xl font-medium text-gray-900">Applications</h1>
-        <a href="/apps/new" class="btn-primary">
-            <svg class="w-5 h-5 mr-1" fill="none" stroke="currentColor" viewBox="0 0 24 24">
-                <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 4v16m8-8H4"/>
-            </svg>
-            New App
-        </a>
+        <div class="flex gap-3">
+            <a href="/backup/import" class="btn-secondary">
+                <svg class="w-4 h-4 mr-1 inline" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                    <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M4 16v1a3 3 0 003 3h10a3 3 0 003-3v-1m-4-8l-4-4m0 0L8 8m4-4v12"/>
+                </svg>
+                Backup / Restore
+            </a>
+            <a href="/apps/new" class="btn-primary">
+                <svg class="w-5 h-5 mr-1" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                    <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M12 4v16m8-8H4"/>
+                </svg>
+                New App
+            </a>
+        </div>
     </div>
 
     {{if .AppStats}}