Add delete marker support to paths

Introduce DeleteMarker struct and associated functions for managing
soft-delete markers within case directories. This enables tracking
deleted cases and supports undo functionality.
feat: Add delete marker support to paths

Introduces a `.deleted` file to mark cases as soft-deleted. The marker
contains a `batch` UUID for grouping deletions and a `deleted_at`
timestamp for ordering.

New functions `is_deleted`, `read_delete_marker`, and
`write_delete_marker` are added to manage this marker. The `locate_case`
function is updated to also consider soft-deleted cases as not found.

server/src/routes/user_web.rs

@@ -166,14 +166,17 @@ pub async fn handle_case_detail(
 }
 
 /// Locate a case directory under `<user_root>/<case_id>/`. Returns `None`
-/// if the directory does not exist (treated as a 404 / IDOR probe by callers).
+/// if the directory does not exist OR is soft-deleted (`.deleted` marker
+/// present). Both are treated as 404 / IDOR probe by callers.
 pub(crate) async fn locate_case(user_root: &Path, case_id: &str) -> Option<std::path::PathBuf> {
     let p = user_root.join(case_id);
-    if tokio::fs::try_exists(&p).await.unwrap_or(false) {
-        Some(p)
-    } else {
-        None
+    if !tokio::fs::try_exists(&p).await.unwrap_or(false) {
+        return None;
     }
+    if crate::paths::is_deleted(&p).await {
+        return None;
+    }
+    Some(p)
 }
 
 /// Scan all of the given user's cases. Returned vec is sorted by most