Add Obsidian vault import feature

- New 'Import Obsidian' action on site settings page
- Upload a zip file of an Obsidian vault to import all notes as posts
- Markdown notes imported with title from filename, published date from
  file timestamp, and body with front-matter stripped
- Images and other attachments saved as Upload records
- New obsimport service handles zip traversal and import logic
- Unit tests for front-matter stripping

Co-authored-by: Shelley <shelley@exe.dev>

views/obsimport/result.html

@@ -0,0 +1,10 @@
+<main class="container">
+  <div>
+    <h5 class="my-4">Import Complete</h5>
+    <div class="alert alert-success">
+      <p class="mb-1">Successfully imported <strong>{{ .result.PostsImported }}</strong> post(s) and <strong>{{ .result.UploadsImported }}</strong> upload(s).</p>
+    </div>
+    <a href="{{ .siteURL }}" class="btn btn-primary">Go to Posts</a>
+    <a href="/sites/{{ .site.ID }}/settings" class="btn btn-secondary ms-2">Back to Settings</a>
+  </div>
+</main>