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.

frontend/src/components/Layout.tsx

@@ -6,6 +6,7 @@ const NAV_ITEMS = [
   { to: "/vehicles", label: "Vehicles" },
   { to: "/bookings", label: "Bookings" },
   { to: "/data-quality", label: "Data Quality" },
+  { to: "/knowledge", label: "Knowledge" },
   { to: "/automation", label: "Automation" },
   { to: "/audit", label: "Audit" },
 ];