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/paths.rs

@@ -1,5 +1,8 @@
 use std::path::{Path, PathBuf};
 
+use serde::{Deserialize, Serialize};
+use uuid::Uuid;
+
 /// Absolute on-disk location of a case directory.
 ///
 /// Single source of truth for the layout `<data_path>/<slug>/<case_id>/`.
@@ -8,3 +11,41 @@ use std::path::{Path, PathBuf};
 pub fn case_dir(data_path: &Path, slug: &str, case_id: &str) -> PathBuf {
     data_path.join(slug).join(case_id)
 }
+
+/// Filename of the soft-delete marker inside a case directory.
+pub const DELETE_MARKER: &str = ".deleted";
+
+/// Soft-delete marker payload. One file per deleted case; all cases removed
+/// in the same user click share the same `batch` UUID. `deleted_at` is an
+/// RFC3339 timestamp — used to find the most recent batch for "Undo last
+/// delete".
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct DeleteMarker {
+    pub batch: Uuid,
+    pub deleted_at: String,
+}
+
+/// True iff the case directory contains a `.deleted` marker file.
+pub async fn is_deleted(case_dir: &Path) -> bool {
+    tokio::fs::try_exists(case_dir.join(DELETE_MARKER))
+        .await
+        .unwrap_or(false)
+}
+
+/// Read and parse the `.deleted` marker. Returns `None` if absent or malformed
+/// (treated identically — a malformed marker still means "deleted" via
+/// `is_deleted`, but cannot participate in batch-undo).
+pub async fn read_delete_marker(case_dir: &Path) -> Option<DeleteMarker> {
+    let bytes = tokio::fs::read(case_dir.join(DELETE_MARKER)).await.ok()?;
+    serde_json::from_slice(&bytes).ok()
+}
+
+/// Write the `.deleted` marker as JSON. Overwrites any existing marker.
+pub async fn write_delete_marker(
+    case_dir: &Path,
+    marker: &DeleteMarker,
+) -> std::io::Result<()> {
+    let bytes = serde_json::to_vec(marker)
+        .map_err(|e| std::io::Error::other(format!("serialize delete marker: {e}")))?;
+    tokio::fs::write(case_dir.join(DELETE_MARKER), bytes).await
+}