Add Obsidian vault import feature (#8)

- 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>

Co-authored-by: exe.dev user <exedev@kernel-leviathan.exe.xyz>
Reviewed-on: #8

views/sitesettings/general.html

@@ -66,5 +66,12 @@
         </div>
       </div>
     </form>
+    <div class="row mb-3">
+      <div class="col-sm-3"></div>
+      <div class="col-sm-9">
+        <a href="/sites/{{ .site.ID }}/import/obsidian" class="btn btn-secondary">Import Obsidian</a>
+        <span class="form-text mx-3">Import posts and attachments from an Obsidian vault zip file.</span>
+      </div>
+    </div>
   </div>
 </main>