feat(groups): tree repos, hierarchy-aware RBAC, admin API

Server-side foundation for Phase-2 groups (no group-owned resources yet):

Shared types:
- GroupId, Group; App gains group_id; AppRole::{precedence,max} for
  folding the highest effective role across the membership chain.

Repos:
- group_repo: tree CRUD with reparent (ancestor-walk cycle guard under a
  coarse instance-wide structural advisory lock; slug frozen; bumps
  structure_version) and delete=RESTRICT (refuses non-empty groups).
- group_members_repo: per-(user, group) role grants, mirroring app_members.

Hierarchy-aware authz (§5.3):
- AuthzRepo gains effective_app_role / effective_group_role (default to
  direct membership / none, so the ~18 existing test stubs are untouched);
  the Postgres impl resolves each via one depth-bounded recursive CTE that
  MAXes the app's own row with every ancestor group_members row.
- can(): the Member path now folds inherited group roles, so a group_admin
  on any ancestor is implicitly app_admin beneath it. New Capability
  variants InstanceCreateGroup / Group{Read,Write,Admin}; group caps carry
  no app_id (bound API keys can't manage groups). 8 new unit tests.

Admin API:
- groups_api: group CRUD + reparent (admin at both source and destination
  parent, §5.6) + per-group members, all capability-gated.
- apps: POST /apps takes an optional parent group (default root); app
  responses carry group_id; my_role now reflects the effective role.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/manager-core/src/lib.rs

@@ -47,6 +47,9 @@ pub mod files_repo;
 pub mod files_service;
 pub mod files_sweep;
 pub mod gc;
+pub mod group_members_repo;
+pub mod group_repo;
+pub mod groups_api;
 pub mod http_service;
 pub mod invoke_service;
 pub mod kv_api;
@@ -165,6 +168,15 @@ pub use files_repo::{FilesConfig, FilesRepo, FilesRepoError, FsFilesRepo};
 pub use files_service::FilesServiceImpl;
 pub use files_sweep::{spawn_files_orphan_sweep, sweep_orphan_tmp_files, SweepStats};
 pub use gc::{spawn_abandoned_gc, spawn_app_user_token_gc, spawn_dead_letter_gc};
+pub use group_members_repo::{
+    GroupMembersRepository, GroupMembersRepositoryError, GroupMembershipDetail, GroupMembershipRow,
+    PostgresGroupMembersRepository,
+};
+pub use group_repo::{
+    GroupChildCounts, GroupRepository, GroupRepositoryError, PostgresGroupRepository,
+    ROOT_GROUP_SLUG,
+};
+pub use groups_api::{groups_router, GroupsApiError, GroupsState};
 pub use http_service::{HttpConfig, HttpServiceImpl};
 pub use kv_api::{kv_admin_router, KvAdminState};
 pub use kv_repo::{KvRepo, KvRepoError, PostgresKvRepo};