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/user_web.rs

@@ -152,9 +152,6 @@ struct MyCasesTemplate {
     slug: String,
     groups: Vec<DateGroup>,
     total: usize,
-    /// Number of cases that "Undo last delete" would restore. 0 hides the
-    /// undo button entirely.
-    undo_count: usize,
     /// True iff the session user is an admin — toggles full case_id display.
     is_admin: bool,
 }
@@ -319,16 +316,11 @@ pub async fn handle_my_cases(
     let cases = scan_user_cases(&config, &user.slug, busy).await;
     let total = cases.len();
     let groups = group_by_utc_date(cases);
-    let undo_count = crate::routes::case_actions::summarize_latest_close_group(&user_root)
-        .await
-        .map(|(_, n)| n)
-        .unwrap_or(0);
     let is_admin = user.is_admin();
     MyCasesTemplate {
         slug: user.slug,
         groups,
         total,
-        undo_count,
         is_admin,
     }
     .render()