Refactor: Simplify case directory structure

The distinction between `open/` and `done/` directories for cases has
been removed.
All cases for a user now reside directly under the user's directory
(e.g., `<data_path>/<slug>/<case_id>/`).

This simplifies path management and eliminates redundant directory
traversals.

Key changes include:
- Removed `open/` and `done/` subdirectories in path resolution.
- Introduced a `paths::case_dir` function as a single source of truth
  for case directory layout.
- Updated various modules (`recovery`, `auth`, `routes`, `transcribe`,
  `tests`) to use the new path structure.
- Adjusted templates to reflect the simplified case status
  representation.

server/templates/case_detail.html

@@ -33,7 +33,7 @@ header form { margin: 0; }
 <div><a class="back" href="/web/cases">&larr; Zurück</a></div>
 <form method="post" action="/web/logout"><button type="submit">Logout</button></form>
 </header>
-<h1>Fall {{ case_id_short }} <span class="status-badge {{ status }}">{{ status }}</span></h1>
+<h1>Fall {{ case_id_short }} {% if has_document %}<span class="status-badge done">ausgewertet</span>{% else %}<span class="status-badge open">offen</span>{% endif %}</h1>
 {% match oneliner %}
 {% when Some with (t) %}
 <div class="oneliner">{{ t }}</div>