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>

backend/app/api/router.py

@@ -14,6 +14,7 @@ from app.api.endpoints import survey
 from app.api.endpoints import admin_survey
 from app.api.endpoints import tree_transfer
 from app.api.endpoints import ai_suggestions
+from app.api.endpoints import kb_accelerator
 
 api_router = APIRouter()
 
@@ -52,3 +53,4 @@ api_router.include_router(survey.router)
 api_router.include_router(admin_survey.router)
 api_router.include_router(tree_transfer.router)
 api_router.include_router(ai_suggestions.router)
+api_router.include_router(kb_accelerator.router)