feat: KB Accelerator — convert KB articles into interactive flows

Full-stack implementation of the KB Accelerator feature that converts
static MSP knowledge base articles into interactive troubleshooting
and procedural flows using AI.

Backend:
- Migrations 054/055: kb_imports, kb_import_nodes tables + plan_limits KB columns
- SQLAlchemy models with relationships and self-referential node hierarchy
- Text extraction service (txt, paste, docx with structural metadata)
- AI conversion service with MSP-specialist prompts for both flow types
- 8 API endpoints: upload, get, list, convert, edit node, commit, delete, quota
- Tier-gated access via plan_limits (free: 3 lifetime, pro/team: unlimited)
- 8 integration tests covering upload, get/list, quota, commit, delete

Frontend:
- TypeScript types and API client for all KB Accelerator endpoints
- Multi-step wizard page: upload → processing → review → success
- Upload screen with paste/file tabs, drag-drop, target type selector
- Two-panel review screen with source highlighting and node cards
- Per-node actions: approve, edit, regenerate, insert, delete
- Confidence color indicators (green/amber/red)
- Sidebar navigation with Sparkles icon
- Code-split lazy-loaded route at /kb-accelerator

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

frontend/src/router.tsx

@@ -41,6 +41,7 @@ const MyAnalyticsPage = lazy(() => import('@/pages/MyAnalyticsPage'))
 const FeedbackPage = lazy(() => import('@/pages/FeedbackPage'))
 const StepLibraryPage = lazy(() => import('@/pages/StepLibraryPage'))
 const AssistantChatPage = lazy(() => import('@/pages/AssistantChatPage'))
+const KBAcceleratorPage = lazy(() => import('@/pages/KBAcceleratorPage'))
 const GuidesHubPage = lazy(() => import('@/pages/GuidesHubPage'))
 const GuideDetailPage = lazy(() => import('@/pages/GuideDetailPage'))
 const AccountSettingsPage = lazy(() => import('@/pages/AccountSettingsPage'))
@@ -153,6 +154,7 @@ export const router = sentryCreateBrowserRouter([
       { path: 'analytics/me', element: page(MyAnalyticsPage) },
       { path: 'feedback', element: page(FeedbackPage) },
       { path: 'step-library', element: page(StepLibraryPage) },
+      { path: 'kb-accelerator', element: page(KBAcceleratorPage) },
       { path: 'assistant', element: page(AssistantChatPage) },
       { path: 'guides', element: page(GuidesHubPage) },
       { path: 'guides/:slug', element: page(GuideDetailPage) },