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/auth.rs

@@ -59,8 +59,7 @@ where
             .ok_or(AppError::Unauthorized)?;
 
         let data_dir = config.data_path.join(slug);
-        tokio::fs::create_dir_all(data_dir.join("open")).await?;
-        tokio::fs::create_dir_all(data_dir.join("done")).await?;
+        tokio::fs::create_dir_all(&data_dir).await?;
 
         Ok(Self {
             slug: slug.clone(),