refactor: replace delete/undo flow with close/reopen endpoints

- POST /web/cases/:id/delete renamed to /close; emits CaseClosed.
- New POST /web/cases/:id/reopen removes the marker; emits CaseReopened.
- Bulk action "delete" renamed to "close"; the shared closed_at timestamp
  still groups the selection for future UI features.
- POST /web/cases/undo-delete removed along with summarize_latest_close_group,
  latest_close_timestamp and restore_close_group. Per-case reopen makes
  the group-undo banner obsolete.
- CaseEventKind::CaseDeleted -> CaseClosed, CaseRestored -> CaseReopened.
- Templates updated to Lucide trash-2 icon, German label "Fall schliessen".
- Tests migrated; delete_watermark_test.rs moved to close_watermark_test.rs.

server/tests/case_page_test.rs

@@ -239,8 +239,8 @@ async fn case_page_empty_case_shows_placeholder() {
         "empty-case placeholder missing"
     );
     assert!(
-        body.contains(&format!(r#"action="/web/cases/{case_id}/delete""#)),
-        "delete form missing on empty case"
+        body.contains(&format!(r#"action="/web/cases/{case_id}/close""#)),
+        "close form missing on empty case"
     );
 }
 
@@ -476,7 +476,7 @@ async fn case_recordings_has_no_action_buttons() {
         "recordings page must not expose the analyze action"
     );
     assert!(
-        !body.contains(&format!(r#"action="/web/cases/{case_id}/delete""#)),
-        "recordings page must not expose the delete action"
+        !body.contains(&format!(r#"action="/web/cases/{case_id}/close""#)),
+        "recordings page must not expose the close action"
     );
 }