feat(email-routing): per-category send-from routing infra + admin matrix

Per PRE-DEPLOY-PLAN § 1.3.7. Lays the foundation for admin-configurable
routing of every outbound email category to either the noreply or
sales sender account.

Pieces shipped:
- `src/lib/services/email-routing.ts` — EmailCategory enum (17
  categories covering every shipped surface), DEFAULT_CATEGORY_ROUTING
  map (auth/notifications/EOI-invite → noreply; brochure/PDF/sales
  send-outs → sales), `resolveSenderForCategory()` + a graceful
  fallback to noreply when the resolved sender is sales but creds
  aren't configured.
- `GET / PATCH /api/v1/admin/email/routing` endpoints — gated on
  `admin.manage_settings`. Returns the routing + sales-availability
  flag + canonical category list.
- `EmailRoutingCard` — matrix UI dropped into /admin/email below the
  sales-email-config card. Per-category dropdown auto-disables the
  `sales` option when the port has no sales SMTP creds; explains the
  state in an amber callout. Save-on-change with toast + "Reset to
  defaults" button.

Setting persisted as `system_settings.email_routing` (JSONB blob).
Followup: opportunistic migration of existing dispatchers (sendEmail,
createSalesTransporter callers) to use `resolveSenderForCategory()` —
the defaults preserve current behavior so this is non-blocking.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/(dashboard)/[portSlug]/admin/email/page.tsx

@@ -4,6 +4,7 @@ import {
 } from '@/components/admin/shared/settings-form-card';
 import { PageHeader } from '@/components/shared/page-header';
 import { SalesEmailConfigCard } from '@/components/admin/sales-email-config-card';
+import { EmailRoutingCard } from '@/components/admin/email-routing-card';
 
 const FIELDS: SettingFieldDef[] = [
   {
@@ -80,6 +81,7 @@ export default function EmailSettingsPage() {
         fields={FIELDS.slice(3)}
       />
       <SalesEmailConfigCard />
+      <EmailRoutingCard />
     </div>
   );
 }