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/backup_import.html

@@ -0,0 +1,62 @@
+{{template "base" .}}
+
+{{define "title"}}Import Backup - µPaaS{{end}}
+
+{{define "content"}}
+{{template "nav" .}}
+
+<main class="max-w-4xl mx-auto px-4 py-8">
+    <div class="mb-6">
+        <a href="/" class="text-primary-600 hover:text-primary-800 inline-flex items-center">
+            <svg class="w-4 h-4 mr-1" fill="none" stroke="currentColor" viewBox="0 0 24 24">
+                <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M15 19l-7-7 7-7"/>
+            </svg>
+            Back to Dashboard
+        </a>
+    </div>
+
+    {{template "alert-success" .}}
+    {{template "alert-error" .}}
+
+    <h1 class="text-2xl font-medium text-gray-900 mb-6">Import Backup</h1>
+
+    <div class="card p-6 mb-6">
+        <h2 class="section-title mb-4">Restore from Backup File</h2>
+        <p class="text-sm text-gray-500 mb-4">
+            Upload a previously exported µPaaS backup file (JSON) to restore app configurations.
+            New apps will be created with fresh SSH keys and webhook secrets.
+            Apps whose names already exist will be skipped.
+        </p>
+        <form method="POST" action="/backup/import" enctype="multipart/form-data">
+            {{ .CSRFField }}
+            <div class="mb-4">
+                <label for="backup_file" class="form-label">Backup File</label>
+                <input type="file" id="backup_file" name="backup_file" accept=".json,application/json"
+                    class="block w-full text-sm text-gray-500
+                    file:mr-4 file:py-2 file:px-4
+                    file:rounded file:border-0
+                    file:text-sm file:font-medium
+                    file:bg-primary-50 file:text-primary-700
+                    hover:file:bg-primary-100
+                    cursor-pointer">
+            </div>
+            <button type="submit" class="btn-primary">Import</button>
+        </form>
+    </div>
+
+    <div class="card p-6">
+        <h2 class="section-title mb-4">Export All Apps</h2>
+        <p class="text-sm text-gray-500 mb-4">
+            Download a backup of all app configurations. This includes app settings,
+            environment variables, labels, volumes, and port mappings.
+            Secrets (SSH keys, webhook tokens) are not included — they are regenerated on import.
+        </p>
+        <a href="/backup/export" 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-4-4l-4 4m0 0l-4-4m4 4V4"/>
+            </svg>
+            Export All Apps
+        </a>
+    </div>
+</main>
+{{end}}