feat(email): Phase 5 — branding chain ext'd with per-port background

Surface hard-coded portnimara.com background image as a per-port
override:

- BrandingShell gains backgroundUrl; renderShell reads from
  branding.backgroundUrl with the existing Port Nimara overhead URL
  as the fallback default.
- getBrandingShell threads the value through from getPortBrandingConfig.
- PortBrandingConfig gains emailBackgroundUrl; SETTING_KEYS adds
  brandingEmailBackgroundUrl mapped to 'branding_email_background_url'.
- /admin/branding page exposes the new field as an image-upload below
  the logo with sizing guidance (1920x1080 JPG, pre-blurred).

This closes the last hard-coded portnimara.com asset URL in the email
shell — every transactional email now fully respects per-port branding
when the admin uploads their own assets. Logo override path was
already in place from R2-H15; the background was the missing piece.

Tests: 1374/1374 passing. tsc clean.

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

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

@@ -45,6 +45,14 @@ const FIELDS: SettingFieldDef[] = [
     imageAspect: 1,
     defaultValue: '',
   },
+  {
+    key: 'branding_email_background_url',
+    label: 'Email background image',
+    description:
+      'Optional blurred photo shown behind the white email card. Leave blank to use the built-in Port Nimara overhead. Recommended: 1920x1080 JPG, pre-blurred to ~20px gaussian so it reads as a soft background even on small clients.',
+    type: 'image-upload',
+    defaultValue: '',
+  },
   {
     key: 'branding_primary_color',
     label: 'Primary color',