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/shared/src/app.rs

@@ -9,7 +9,7 @@ use chrono::{DateTime, Utc};
 use serde::{Deserialize, Serialize};
 use uuid::Uuid;
 
-use crate::AppId;
+use crate::{AppId, GroupId};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct App {
@@ -20,6 +20,9 @@ pub struct App {
     pub slug: String,
     pub name: String,
     pub description: Option<String>,
+    /// Parent group in the org tree (Phase 2). Every app has a parent
+    /// from day one (§9 backfill seeds the instance root group).
+    pub group_id: GroupId,
     pub created_at: DateTime<Utc>,
     pub updated_at: DateTime<Utc>,
 }