Fix #145: tree membership management (list / add / role / remove)

TreeMembership was enforced on every read/write but had no API or UI to manage
members — trees were effectively single-user, breaking full-CRUD (NN#8).

Backend (/trees/{id}/members): list (members only — the list exposes emails, so
non-members never see it, even on public trees); add an existing user by email
(owner only, 404 if no such account, 409 if already a member); PATCH role;
DELETE. A tree must always keep ≥1 owner (demote/remove of the sole owner → 409).
All changes audited.

Frontend: a Members page (owner gets add-by-email + per-member role select +
remove; others see a read-only list) and a sidebar entry.

Test covers the full lifecycle + every guard. Suite 77 passed.

Closes #145

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

frontend/components/app-sidebar.tsx

@@ -11,6 +11,7 @@ import {
   Network,
   Settings,
   Sparkles,
+  UserPlus,
   Users,
 } from "lucide-react";
 import Link from "next/link";
@@ -137,6 +138,12 @@ export function AppSidebar({ onNavigate }: { onNavigate?: () => void }) {
             icon={Sparkles}
             active={pathname.startsWith(`/trees/${treeId}/cleanup`)}
           />
+          <Item
+            href={`/trees/${treeId}/members`}
+            label="Members"
+            icon={UserPlus}
+            active={pathname.startsWith(`/trees/${treeId}/members`)}
+          />
           <Item
             href={`/trees/${treeId}/recovery`}
             label="Recovery"