perf(citation-verification): cache view + parallelize backend retrieval

The "אימות פסיקה" tab reloaded its data on every tab revisit and the first
load took 1.5–3s. Two independent causes, two fixes:

Frontend (web-ui/src/lib/api/citation-verification.ts):
- staleTime 10s → 5min and refetchOnMount: false. Radix Tabs unmounts inactive
  TabsContent, so switching back used to trigger a full rebuild once the 10s
  window elapsed. Verify mutations still invalidate the query, so edits pull
  fresh data.

Backend (mcp-server/.../case_citation_verification.py):
- The per-argument loop ran a Voyage embed + vector search + batched authority
  query sequentially (N args × ~300ms waterfall). Extracted to _fetch() and
  fanned out with asyncio.gather — order preserved, n_verified counter and
  per-argument seen-sets unchanged. First load ~5× faster on multi-argument cases.

Pure read/assembly — no new parallel path (G2), no silent error swallowing
(search/radar still log warnings). Invariants: INV-DIG1/INV-AH unaffected
(still read-only, never cites).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web-ui/src/lib/api/citation-verification.ts

@@ -84,7 +84,13 @@ export function useCitationVerification(caseNumber: string | undefined) {
         { signal },
       ),
     enabled: Boolean(caseNumber),
-    staleTime: 10_000,
+    // The view is an expensive synthesized build (per-argument Voyage embed +
+    // vector search on the backend). Keep it fresh for 5 min and do NOT refetch
+    // on remount — Radix Tabs unmounts inactive TabsContent, so every tab switch
+    // back used to trigger a full rebuild. Verify mutations invalidate the query,
+    // so edits still pull fresh data.
+    staleTime: 5 * 60_000,
+    refetchOnMount: false,
   });
 }