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/App.tsx

@@ -11,6 +11,7 @@ import { BookingDetail } from "./pages/BookingDetail";
 import { DataQuality } from "./pages/DataQuality";
 import { DataQualityIssueDetail } from "./pages/DataQualityIssueDetail";
 import { Automation } from "./pages/Automation";
+import { Knowledge } from "./pages/Knowledge";
 import { Audit } from "./pages/Audit";
 
 export function App() {
@@ -33,6 +34,7 @@ export function App() {
           <Route path="/data-quality" element={<DataQuality />} />
           <Route path="/data-quality/:publicRef" element={<DataQualityIssueDetail />} />
           <Route path="/automation" element={<Automation />} />
+          <Route path="/knowledge" element={<Knowledge />} />
           <Route path="/audit" element={<Audit />} />
         </Route>
         <Route path="/" element={<Navigate to="/dashboard" replace />} />