feat(menus): tree endpoint + get-by-id + icon + admin-ui field names

[jlc488]

First PR in a small series closing contract gaps between the admin-ui PRs (devslab-kr/devslab-kit-admin-ui#2–#7) and the backend on main. The admin-ui's MenusView calls three things this controller didn't expose, plus a column the entity didn't have.

Summary

Endpoints

• GET /admin/api/v1/menus/tree?tenantId=... — new. Returns the nested tree the TreeTable component needs (each root carries its descendants under children, sorted by displayOrder at every level). Built in-controller from the existing flat repo query — single round-trip.
• GET /admin/api/v1/menus/{id} — new. Returns 404 on miss.

DTO wire-shape (matches the admin-ui)

• CreateMenuRequest / UpdateMenuRequest — fields renamed to what the admin UI sends: displayOrder (was sortOrder), requiredPermission (was requiredPermissionCode), plus the new icon. Validation constraints preserved.
• MenuResponse — two factory methods. flat(entity) for the existing list + new get-by-id (children = null). withChildren(entity, list) for the tree endpoint. Wire field names also follow the UI.

Entity / schema

• PlatformMenuEntity — new icon field (nullable, VARCHAR(64)). All-args constructor extended.
• V8__platform_menu_icon.sql — ALTER TABLE platform_menu ADD COLUMN icon VARCHAR(64);. Existing rows unaffected; icon-less items render text-only on the UI.

Service layer

• MenuAdminService.create / update — new icon parameter; partial-update semantics on update (icon updates only if non-null), matching the existing pattern for label/path/sortOrder.
• MenuAdminService.findById — new read used by GET /menus/{id}.

Tree propagation

• MenuItem API record — new icon field.
• MenuTreeBuilder reads it from the entity.
• PermissionBasedMenuFilter preserves it when pruning by user permissions.
• DiagnosticsController.filterByCodes does the same when computing menu visibility.

Test fixture

• MenuTreeBuilderTest entity factory updated to the new constructor (icon = null).

Gotcha worth flagging

The V8 migration comment intentionally avoids ${...} even though it's just a code-comment example. Flyway's PlaceholderReplacingReader scans SQL comments too — my first attempt had pi pi-${icon} in a -- line and the sample-app integration tests blew up with org.flywaydb.core.api.FlywayException. Easy fix once you know.

Test plan

• ./gradlew build green (55 tasks, all sample-app + menu-core tests pass)
• Wire up the admin-ui PR stack against this and exercise the Menus page end-to-end (TreeTable renders, add-child / edit (with icon) / delete all hit the right endpoints)

What's next in this series

Other gap closures still queued (separate PRs): AuditLogs (paging + filters + field rename), Policies (PolicyDescriptor list + nested test body), Tenants (status enum + createdAt), Diagnostics + Settings field names.