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/cache-prune.ts

@@ -0,0 +1,41 @@
+import { createChildLogger } from "@/shared/logger/index";
+import { config } from "../../shared/config/index.js";
+import { deleteExpiredQdrantPoints } from "./qdrantClient.js";
+import { pruneExpiredTexts } from "./textCacheStore.js";
+
+const logger = createChildLogger("cache-prune");
+
+/** Expired-verdict sweep cadence. */
+const CACHE_PRUNE_INTERVAL_MS = 6 * 60 * 60 * 1000; // every 6 hours
+
+let lastCachePruneAt = 0;
+
+/**
+ * Cache hygiene: purge expired moderation verdicts from Postgres and Qdrant.
+ *
+ * Expired entries are never reused (read filters check `expires_at`) but they
+ * accumulate forever without a sweep. Called from the recovery interval; the
+ * 6-hour throttle keeps it to one sweep per window.
+ */
+export function runCachePruneIfDue(now: number = Date.now()): void {
+  if (now - lastCachePruneAt < CACHE_PRUNE_INTERVAL_MS) return;
+  lastCachePruneAt = now;
+
+  Promise.all([pruneExpiredTexts(), deleteExpiredQdrantPoints()])
+    .then(([pgDeleted, qdDeleted]) => {
+      if (pgDeleted > 0 || qdDeleted > 0) {
+        logger.info(
+          { pgDeleted, qdDeleted },
+          "Expired moderation cache pruned",
+        );
+      }
+    })
+    .catch((err: unknown) => {
+      logger.warn({ error: String(err) }, "Moderation cache prune failed");
+    });
+}
+
+/** Reset the throttle window (tests). */
+export function resetCachePruneState(): void {
+  lastCachePruneAt = 0;
+}