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/src/routes/mod.rs

@@ -43,17 +43,17 @@ pub fn api_router() -> Router<AppState> {
             post(case_actions::handle_analyze_case),
         )
         .route(
-            "/web/cases/{case_id}/delete",
-            post(case_actions::handle_delete_case),
+            "/web/cases/{case_id}/close",
+            post(case_actions::handle_close_case),
+        )
+        .route(
+            "/web/cases/{case_id}/reopen",
+            post(case_actions::handle_reopen_case),
         )
         .route(
             "/web/cases/{case_id}/reset",
             post(case_actions::handle_reset_case),
         )
-        .route(
-            "/web/cases/undo-delete",
-            post(case_actions::handle_undo_delete),
-        )
         .route("/web/cases/bulk", post(bulk::handle_bulk_action))
         .route(
             "/web/audio/{user}/{case_id}/{filename}",