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/Cargo.toml

@@ -14,7 +14,7 @@ tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
 tracing-appender = "0.2"
 dotenvy = "0.15"
-uuid = { version = "1", features = ["v4"] }
+uuid = { version = "1", features = ["v4", "serde"] }
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
 toml = "0.8"