Add admin-only case reset functionality

Introduce a new "reset" action for bulk operations and a dedicated
endpoint for individual case resets. These features allow administrators
to revert a case to its raw audio state by deleting derived artifacts
like transcripts, analysis input, and documents. The functionality also
includes renaming `.m4a.failed` files back to `.m4a` to re-trigger
transcription.

This commit also:
- Adds a `reset_case_artefacts` helper function to `case_actions.rs`.
- Implements the `handle_reset_case` endpoint.
- Adds the "Reset" button to the UI for administrators.
- Includes unit and integration tests to verify the new functionality
  and access control.

server/src/routes/mod.rs

@@ -29,6 +29,10 @@ pub fn api_router() -> Router<AppState> {
             "/web/cases/{case_id}/delete",
             post(case_actions::handle_delete_case),
         )
+        .route(
+            "/web/cases/{case_id}/reset",
+            post(case_actions::handle_reset_case),
+        )
         .route(
             "/web/cases/undo-delete",
             post(case_actions::handle_undo_delete),