Org structure: divisions → products/services → teams + custom model base URL

The object spine becomes definable (data model was designed-for from day one):
- Division and Product entities (Product carries kind: Product|Service, optional DivisionId);
  Team gains nullable ProductId — pre-structure teams keep working. AddDivisionsAndProducts
  migration; org-scoped validation; owner-only writes (audited); list endpoints.
- /structure page: define divisions, products/services (with division), teams (under a
  product). Org chart now renders the full spine — org → divisions → products → teams →
  seats — with parentless layers linking up to the org.
- BYOK custom URL: the SeatsPage model-connection form gains a Base URL field (provider
  list: stub/openai/ollama/vllm/custom). Backend already supported it end to end —
  ApiConfig.Endpoint flows into the OpenAI-compatible adapter ({base}/v1/chat/completions),
  so any OpenAI-compatible gateway or self-hosted model works; the config list shows it.

Verified: ArchitectureTests 8/8, IntegrationTests 45/45 (new OrgStructureTests: spine
creation, kind tags, org-scoped validation 400s, Member 403), client build green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

client/src/components/AppShell.tsx

@@ -2,6 +2,7 @@ import type { ReactNode } from 'react'
 import { Link, useLocation } from 'react-router'
 import {
   Bot,
+  Boxes,
   ChartColumn,
   Gauge,
   Inbox,
@@ -42,6 +43,7 @@ export function AppShell({ children }: { children: ReactNode }) {
           <NavItem icon={ShieldCheck} label="Review inbox" to="/reviews" />
           <NavItem icon={Bot} label="AI seats" to="/seats" />
           <NavItem icon={Network} label="Org chart" to="/org" />
+          <NavItem icon={Boxes} label="Structure" to="/structure" />
           <NavItem icon={Users} label="Members" to="/members" />
           <NavItem icon={Gauge} label="Performance" to="/performance" />
           <NavItem icon={ChartColumn} label="Analytics" to="/analytics" />