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/src/modules/health/moderationMetrics.ts

@@ -0,0 +1,80 @@
+/**
+ * moderationMetrics.ts
+ *
+ * Prometheus metrics for AI moderation pipeline.
+ * Defined in backend (where prom-client is installed + /api/metrics endpoint).
+ */
+import { Counter, Histogram, register } from "prom-client";
+
+// ── LLM Call Metrics ──
+export const llmCallsTotal = new Counter({
+  name: "moderation_llm_calls_total",
+  help: "Total LLM moderation calls",
+  labelNames: ["path", "model"] as const,
+});
+
+export const llmCallDuration = new Histogram({
+  name: "moderation_llm_call_duration_ms",
+  help: "LLM moderation call duration (ms)",
+  labelNames: ["path", "status"] as const,
+  buckets: [500, 1000, 2000, 5000, 10000, 20000, 30000, 60000, 120000],
+});
+
+export const llmTokensTotal = new Counter({
+  name: "moderation_llm_tokens_total",
+  help: "Total tokens consumed by LLM moderation",
+  labelNames: ["type"] as const,
+});
+
+// ── Cache Metrics ──
+export const moderationCacheHits = new Counter({
+  name: "moderation_cache_hits_total",
+  help: "Moderation cache hits",
+  labelNames: ["layer"] as const,
+});
+
+export const moderationCacheMisses = new Counter({
+  name: "moderation_cache_misses_total",
+  help: "Moderation cache misses",
+  labelNames: ["layer"] as const,
+});
+
+// ── Media Analysis Metrics ──
+export const mediaAnalysesTotal = new Counter({
+  name: "moderation_media_analyses_total",
+  help: "Media analyses performed",
+  labelNames: ["type"] as const,
+});
+
+export const mediaDownloadDuration = new Histogram({
+  name: "moderation_media_download_duration_ms",
+  help: "Media download duration (ms)",
+  labelNames: ["source"] as const,
+  buckets: [100, 500, 1000, 2000, 5000, 10000, 30000],
+});
+
+// ── Batch & Error Metrics ──
+export const moderationBatchSize = new Histogram({
+  name: "moderation_batch_size",
+  help: "Messages per batch",
+  labelNames: ["path"] as const,
+  buckets: [1, 5, 10, 20, 50, 100],
+});
+
+export const moderationErrors = new Counter({
+  name: "moderation_errors_total",
+  help: "Moderation errors",
+  labelNames: ["type"] as const,
+});
+
+export const searxngCalls = new Counter({
+  name: "moderation_searxng_calls_total",
+  help: "SearXNG search calls",
+  labelNames: ["status"] as const,
+});
+
+export const autoDeleteActions = new Counter({
+  name: "moderation_auto_delete_total",
+  help: "Auto-delete actions",
+  labelNames: ["action"] as const,
+});