feat(demo): make return, data-quality and knowledge flows demo-legible

Fixes a real honesty bug in Knowledge.tsx (body copy named "RAGcore" while
the active provider is the demo one) and a second real bug discovered
while fixing it: the brief's suggested Dutch questions would silently
return "insufficient evidence" against the English-only demo knowledge
base -- verified empirically and fixed by keeping suggested questions in
English. The return flow now pre-fills the odometer-regression scenario's
suspicious reading instead of asking a visitor to invent one, and links
to automation/audit after committing. Data-quality issues get a shared
plain-language "what's wrong / why it matters" explainer per rule type,
a post-resolution confirmation with audit/vehicle links, and a "demo
scenario's only" list filter. Also fixes a real async race where the
odometer pre-fill could clobber text a visitor had already started typing.

frontend/src/pages/DataQuality.tsx

@@ -24,6 +24,7 @@ export function DataQuality() {
   const [scanError, setScanError] = useState<string | null>(null);
   const [scanResult, setScanResult] = useState<ScanResult | null>(null);
   const [confirmingScan, setConfirmingScan] = useState(false);
+  const [demoScenariosOnly, setDemoScenariosOnly] = useState(false);
 
   const load = useCallback(() => {
     if (user?.role !== "operations_manager") return;
@@ -67,6 +68,11 @@ export function DataQuality() {
   }
 
   const scanTotal = scanResult ? Object.values(scanResult.created).reduce((a, b) => a + b, 0) : 0;
+  const visibleIssues = issues
+    ? demoScenariosOnly
+      ? issues.filter((i) => i.public_ref.startsWith("DQ-DEMO-"))
+      : issues
+    : [];
 
   return (
     <div className="page">
@@ -126,14 +132,25 @@ export function DataQuality() {
             ))}
           </select>
         </label>
+        <label className="checkbox-label">
+          <input
+            type="checkbox"
+            checked={demoScenariosOnly}
+            onChange={(e) => setDemoScenariosOnly(e.target.checked)}
+          />
+          Demo scenario's only
+        </label>
       </form>
 
       {error && <ErrorState message={error} />}
       {!error && !issues && <LoadingState label="Loading quality workbench…" />}
       {issues && issues.length === 0 && <EmptyState icon="check" title="Queue is clear" detail="No issues match the current filters." />}
+      {issues && issues.length > 0 && visibleIssues.length === 0 && (
+        <EmptyState icon="check" title="No demo-scenario issues match" detail="Uncheck 'Demo scenario's only' to see the full queue." />
+      )}
 
-      {issues && issues.length > 0 && (
-        <div className="table-shell"><div className="table-meta"><span>{issues.length} issues</span><span>Evidence-backed detection</span></div><table className="data-table">
+      {visibleIssues.length > 0 && (
+        <div className="table-shell"><div className="table-meta"><span>{visibleIssues.length} issues</span><span>Evidence-backed detection</span></div><table className="data-table">
           <caption className="visually-hidden">Data-quality issues</caption>
           <thead>
             <tr>
@@ -145,7 +162,7 @@ export function DataQuality() {
             </tr>
           </thead>
           <tbody>
-            {issues.map((i) => (
+            {visibleIssues.map((i) => (
               <tr key={i.public_ref}>
                 <th scope="row" data-label="Reference">
                   <Link to={`/data-quality/${i.public_ref}`}>{i.public_ref}</Link>