refactor(gateway): split bootstrap + aiAnalyzer, add module barrels

app/:
- bootstrap.ts 277 -> 145 lines: config guard, DB connect, client debug
  logging and startup order are now named steps with a comment header
- lifecycle.ts (new): everything wired on the Discord 'ready' hook, in
  explicit order (inject broadcaster -> register listeners -> start workers)
- process-guards.ts (new): SIGINT/SIGTERM/uncaughtException/unhandledRejection
  in ONE place, using isTransientStreamError() instead of two duplicated
  inline code lists
- metrics-collector.ts (new): AI pipeline Prometheus gauges

modules/:
- ai-moderation/index.ts + message-capture/index.ts (new): public facades so
  app/ never reaches into internal files
- aiAnalyzer.ts 317 -> 146 lines: pure entry API; skip-verdict recording
  extracted into recordSkip()
- recovery-worker.ts (new): stranded-message recovery + stale lane/CB pruning
- cache-prune.ts (new): 6h expired-verdict sweep, throttled + resettable
- drop 3 dead re-exports (pickBatchWithinBudget/onCircuitBreakerAlert/
  getConversationKey) whose consumers import the origin files directly

No behavior change. typecheck + lint + 138 tests green; nix build OK.

services/discord-gateway/src/modules/ai-moderation/index.ts

@@ -0,0 +1,35 @@
+/**
+ * Public surface of the AI-moderation module.
+ *
+ * The module has ~50 internal files; callers outside it (app/, tests, other
+ * modules) should import from THIS barrel so internal files can be moved
+ * without touching call sites.
+ *
+ * Deep imports remain valid inside the module itself.
+ */
+
+export type {
+  AIRecommendedAction,
+  AISeverity,
+  AIStatus,
+  AnalysisQueueStatus,
+  AnalysisResult,
+} from "../../shared/moderation-types.js";
+// ── Entry API: queueing, status, recovery worker ──────────────────────────
+export {
+  getAnalysisQueueStatus,
+  queueConversationAnalysis,
+  queueMessageAnalysis,
+  startPendingAIAnalysisWorker,
+} from "./aiAnalyzer.js";
+// ── Worker pools (app/metrics-collector reads their live thread counters) ──
+export {
+  getConversationKey,
+  mediaWorkerPool,
+  textWorkerPool,
+} from "./circuitBreaker.js";
+// ── Pipeline state hooks the bootstrap injects into ───────────────────────
+export {
+  setModerationClient,
+  setSharedEventBroadcaster,
+} from "./moderationState.js";