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/app/process-guards.ts

@@ -0,0 +1,61 @@
+import { errorMessage, isTransientStreamError } from "@/shared/errors/index.js";
+import type { Logger } from "@/shared/logger/index.js";
+import type { GracefulShutdown } from "./shutdown.js";
+
+/**
+ * Process-level signal + error guards.
+ *
+ * Extracted from bootstrap so the "what keeps the gateway alive vs what
+ * shuts it down" policy lives in exactly one place.
+ *
+ * Policy: transient stream-teardown failures (EPIPE / ERR_STREAM_DESTROYED /
+ * ERR_STREAM_WRITE_AFTER_END / ECONNRESET) are logged and IGNORED — crashing
+ * the gateway on them (voice stop/disconnect races, a child process stdin
+ * closed while we still write) takes the whole bot offline mid-operation.
+ * Anything else is a real bug: log with stack and shut down cleanly.
+ */
+export function registerProcessGuards(
+  logger: Logger,
+  gracefulShutdown: GracefulShutdown,
+): void {
+  process.on("SIGINT", () => {
+    gracefulShutdown("SIGINT");
+  });
+
+  process.on("SIGTERM", () => {
+    gracefulShutdown("SIGTERM");
+  });
+
+  process.on("uncaughtException", (err) => {
+    if (isTransientStreamError(err)) {
+      logger.warn(
+        { error: err },
+        "Uncaught transient stream error — continuing",
+      );
+      return;
+    }
+    logger.error(
+      {
+        err,
+        errorMsg: errorMessage(err),
+        stack: err?.stack,
+      },
+      "Uncaught exception",
+    );
+    gracefulShutdown("uncaughtException");
+  });
+
+  process.on("unhandledRejection", (reason) => {
+    const err =
+      reason instanceof Error ? reason : new Error(String(reason ?? "unknown"));
+    if (isTransientStreamError(err)) {
+      logger.warn(
+        { error: err },
+        "Unhandled rejection transient stream error — continuing",
+      );
+      return;
+    }
+    logger.error({ error: err, reason: String(reason) }, "Unhandled rejection");
+    gracefulShutdown("unhandledRejection");
+  });
+}