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/src/transcribe/recovery.rs

@@ -4,7 +4,7 @@ use tracing::{info, warn};
 
 use super::{TranscribeJob, TranscribeSender};
 
-/// Walk `data_path/*/open/*/*.m4a` and enqueue every recording that does not
+/// Walk `data_path/*/*/*.m4a` and enqueue every recording that does not
 /// yet have a `.transcript.txt` sibling.
 ///
 /// Intended to run once at startup (spawn as a task — sends may back-pressure
@@ -29,7 +29,7 @@ pub async fn scan_and_enqueue(data_path: &Path, tx: &TranscribeSender) {
     }
 }
 
-/// Walks `$data_path/<slug>/open/<case>/*.m4a` and returns `(slug, audio_path)`
+/// Walks `$data_path/<slug>/<case>/*.m4a` and returns `(slug, audio_path)`
 /// tuples for every recording without a `.transcript.txt` sibling. The slug
 /// comes from the top-level user directory name — that's the only ownership
 /// signal after a restart (auth context is gone).
@@ -45,8 +45,7 @@ async fn collect_pending(data_path: &Path) -> Vec<(String, PathBuf)> {
         let Some(slug) = user_entry.file_name().to_str().map(str::to_owned) else {
             continue;
         };
-        let open_dir = user_entry.path().join("open");
-        let mut cases = match tokio::fs::read_dir(&open_dir).await {
+        let mut cases = match tokio::fs::read_dir(user_entry.path()).await {
             Ok(r) => r,
             Err(_) => continue,
         };