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/auth.rs

@@ -98,6 +98,30 @@ impl AppRole {
             _ => None,
         }
     }
+
+    /// Authority rank: higher = more authority. Used to fold the highest
+    /// effective role across an app's own membership and every ancestor
+    /// group membership (hierarchy-aware RBAC). Defined explicitly rather
+    /// than via a derived `Ord` because the variant declaration order
+    /// (`AppAdmin` first) is the reverse of authority order.
+    #[must_use]
+    pub const fn precedence(self) -> u8 {
+        match self {
+            Self::AppAdmin => 3,
+            Self::Editor => 2,
+            Self::Viewer => 1,
+        }
+    }
+
+    /// The more-authoritative of two roles. `app_admin > editor > viewer`.
+    #[must_use]
+    pub fn max(self, other: Self) -> Self {
+        if self.precedence() >= other.precedence() {
+            self
+        } else {
+            other
+        }
+    }
 }
 
 /// API-key scope. Exactly seven values; new scopes need a blueprint