M5: implement RAGcore knowledge integration

KnowledgeProvider protocol with a deterministic TF-IDF-weighted extractive demo provider (never generative, always cites real excerpts) and a RAGcore HTTP adapter that degrades cleanly to unavailable. Knowledge nav + chat-style Q&A UI with source cards and honest grounded/insufficient/unavailable states. 57 backend tests passing, ruff clean. Fixed a real relevance bug (generic terms like "vehicle" crowding out distinctive matches) via IDF weighting, found by testing the actual S6 scenario. Verified end-to-end in the browser: grounded damage question cites both expected procedures; unrelated question honestly returns insufficient evidence with no fabrication.

backend/app/main.py

@@ -12,6 +12,7 @@ from app.api.routers import (
     data_quality,
     demo,
     integrations,
+    knowledge,
     vehicles,
     workflows,
 )
@@ -84,3 +85,4 @@ app.include_router(audit.router)
 app.include_router(data_quality.router)
 app.include_router(workflows.router)
 app.include_router(integrations.router)
+app.include_router(knowledge.router)