refactor(ai): replace SearXNG with Wikipedia adapter for analysis enrichment

- Add wikipediaClient.ts: native fetch to Wikipedia REST/Action APIs
  (search + summary), no extra npm dependency.
- Extract shared Redis cache into cacheStore.ts (decoupled from search).
- Term glossary now uses wikipediaSummary for direct article lookup.
- Remove searxngSearch.ts entirely; drop SEARXNG_BASE_URL config,
  add WIKIPEDIA_LANG / WIKIPEDIA_TIMEOUT_MS.
- Rename backend searxngCalls metric to webSearchCalls.

services/backend/src/modules/health/moderationMetrics.ts

@@ -67,9 +67,9 @@ export const moderationErrors = new Counter({
   labelNames: ["type"] as const,
 });
 
-export const searxngCalls = new Counter({
-  name: "moderation_searxng_calls_total",
-  help: "SearXNG search calls",
+export const webSearchCalls = new Counter({
+  name: "moderation_websearch_calls_total",
+  help: "Wikipedia web-search calls",
   labelNames: ["status"] as const,
 });