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/my_cases.html

@@ -11,8 +11,8 @@ header form { margin: 0; }
 .case:hover { background: #f6f6f6; }
 .case h2 { margin: 0 0 0.3em 0; font-size: 1em; }
 .case small { color: #666; font-family: monospace; }
-.status-open { border-left: 4px solid #4a90e2; }
-.status-done { border-left: 4px solid #7ed321; }
+.case.open { border-left: 4px solid #4a90e2; }
+.case.done { border-left: 4px solid #7ed321; }
 .oneliner { font-size: 1em; color: #333; margin: 0.2em 0; }
 section { margin: 1.5em 0; }
 section h2 { font-size: 1.1em; color: #555; border-bottom: 1px solid #ddd; padding-bottom: 0.2em; }
@@ -29,31 +29,12 @@ section h2 { font-size: 1.1em; color: #555; border-bottom: 1px solid #ddd; paddi
 </header>
 
 <section>
-<h2>Offen ({{ open.len() }})</h2>
-{% if open.is_empty() %}
-<p class="empty">Keine offenen Fälle.</p>
+<h2>Fälle ({{ cases.len() }})</h2>
+{% if cases.is_empty() %}
+<p class="empty">Keine Fälle.</p>
 {% else %}
-{% for case in open %}
-<a class="case status-{{ case.status }}" href="/web/cases/{{ case.case_id }}">
-<h2>{{ case.case_id_short }} — {{ case.recordings.len() }} Aufnahme(n){% if case.has_document %}<span class="label done-doc">ausgewertet</span>{% else if case.analyzing %}<span class="label analyzing">wird analysiert</span>{% endif %}</h2>
-{% match case.oneliner %}
-{% when Some with (t) %}
-<div class="oneliner">{{ t }}</div>
-{% when None %}
-{% endmatch %}
-<small>{{ case.most_recent }}</small>
-</a>
-{% endfor %}
-{% endif %}
-</section>
-
-<section>
-<h2>Abgeschlossen ({{ done.len() }})</h2>
-{% if done.is_empty() %}
-<p class="empty">Keine abgeschlossenen Fälle.</p>
-{% else %}
-{% for case in done %}
-<a class="case status-{{ case.status }}" href="/web/cases/{{ case.case_id }}">
+{% for case in cases %}
+<a class="case {% if case.has_document %}done{% else %}open{% endif %}" href="/web/cases/{{ case.case_id }}">
 <h2>{{ case.case_id_short }} — {{ case.recordings.len() }} Aufnahme(n){% if case.has_document %}<span class="label done-doc">ausgewertet</span>{% else if case.analyzing %}<span class="label analyzing">wird analysiert</span>{% endif %}</h2>
 {% match case.oneliner %}
 {% when Some with (t) %}