style: fmt + clippy cleanup after Phase C

cargo fmt regroups; three clippy fixes:
- F-S-006: hoist MAX_API_KEY_CANDIDATES to module scope to satisfy
  clippy::items_after_statements.
- F-S-009: use map_or instead of map().unwrap_or() per
  clippy::map_unwrap_or.
- F-P-012: replace `|c| c.encode()` closure with the method itself
  per clippy::redundant_closure_for_method_calls.

No behavior change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/manager-core/src/users_service.rs

@@ -587,7 +587,10 @@ impl UsersService for UsersServiceImpl {
             .collect();
         Ok(UsersListPage {
             items,
-            next_cursor: page.next_cursor.as_ref().map(|c| c.encode()),
+            next_cursor: page
+                .next_cursor
+                .as_ref()
+                .map(crate::app_user_repo::ListCursor::encode),
         })
     }