Add service admin groups, combined settings pages, single Settings button

- Auto-create {service-id}-admin groups at startup (group_bootstrap.py)
- get_service_admin() dep: grants access to superusers OR service group members
- /api/settings/ai and /api/settings/documents/limits now allow service admins
- AI service exposes /plugin/manifest (ai-service-admin access group)
- DocServiceSettingsPage: combined upload limits + watch directory on one page
- ServiceAdminRoute in frontend guards new /apps/documents/settings and /apps/ai/settings
- Single Settings button per app card (visible to admins and service group members)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -27,3 +27,4 @@ docker-compose.feat-*.yml
 
 # Don't sync .un files
 *.un~
+dev-watch/**/*.pdf