refactor: split llmModerationClient.ts + add tests + metrics

## Split llmModerationClient.ts (2103 → 3 files)
- **moderationBuilders.ts** (67 lines) — shared: escapeXml, getAnalysisContent, buildReferenceXml
- **mediaAnalysisClient.ts** (656 lines) — vision analysis with multi-layer LRU/DB/phash caching, image/video download, ffmpeg frame extraction, prepareMediaMessage
- **moderationOrchestrator.ts** (998 lines) — callModerationLLM, runTextOnlyBatch, runMediaBatch, runModerationAnalysis, runSimpleTextFallback
- **llmModerationClient.ts** (30 lines) — re-export bridge (backward compat)

No import changes needed — aiAnalysisWorker.ts still imports from llmModerationClient.js.

## Unit tests (backend)
- vitest.config.ts + e2e.test.ts with 9 tests against production:
  - health, metrics, dashboard/stats, recordings, config, auth, guilds, negative (404/400)

## Monitoring metrics
- moderationMetrics.ts in backend health module:
  - LLM call count/duration/tokens
  - Cache hit/miss per layer
  - Media analysis count/download duration
  - Batch size distribution, errors, SearXNG, auto-delete

services/backend/vitest.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    globals: true,
+    environment: "node",
+    include: ["src/**/*.test.ts"],
+    testTimeout: 15000,
+  },
+});