audit(gateway): fix dead /metrics endpoint, raise OOM-prone MemoryMax, trim DB pool

- gateway-metrics: collectors now run per scrape so Prometheus sees real
  data (process memory/uptime + live AI-analysis pipeline gauges) instead
  of an always-empty stub. bootstrap registers the pipeline collectors.
- systemd: MemoryMax 512M -> 1G (live RSS ~500MiB, peak 508MiB; 512M left
  ~2% headroom and risked an OOM-kill restart; host has 8GB free).
- config: POSTGRES_POOL_MIN 2 -> 0 so main + 4 Piscina worker threads don't
  hold ~10 permanently-open idle pg connections against PgBouncer.
- docs: rewrite stale ARCHITECTURE.md / MODULE_STRUCTURE.md (winston ->
  pino, removed mock-crc/indonesianTextNormalizer, renamed
  aiAnalysisWorker/llmModerationClient).

Verified: tsc clean, 129 vitest pass, biome clean on changed files.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

services/discord-gateway/src/app/bootstrap.ts

@@ -3,7 +3,11 @@ import { inArray, lt } from "drizzle-orm";
 import type { NodePgDatabase } from "drizzle-orm/node-postgres";
 import { ConfigError, DatabaseError } from "@/shared/errors/index";
 import { createChildLogger } from "@/shared/logger/index";
-import { startPendingAIAnalysisWorker } from "../modules/ai-moderation/aiAnalyzer.js";
+import {
+  getAnalysisQueueStatus,
+  startPendingAIAnalysisWorker,
+} from "../modules/ai-moderation/aiAnalyzer.js";
+import { workerPool } from "../modules/ai-moderation/circuitBreaker.js";
 import { registerChannelTopicCapture } from "../modules/channel-topic/index.js";
 import { CommandHandler } from "../modules/command-handler/commandHandler.js";
 import {
@@ -11,6 +15,8 @@ import {
   RedisEventPublisher,
 } from "../modules/event-broadcaster/index.js";
 import {
+  registerCollector,
+  setGauge,
   startMetricsServer,
   stopMetricsServer,
 } from "../modules/gateway-metrics/index.js";
@@ -340,6 +346,42 @@ export async function initializeDiscordGateway() {
     gracefulShutdown("unhandledRejection");
   });
 
+  // ── Metrics: register live pipeline collectors before starting server ──
+  // These refresh on every scrape so Prometheus sees real AI-analysis
+  // queue depth, concurrency, and DB pool state instead of an empty stub.
+  registerCollector(() => {
+    if (!config.AI_ANALYSIS_ENABLED) return;
+    try {
+      const status = getAnalysisQueueStatus();
+      setGauge("ai_analysis_queued_conversations", status.queuedConversations);
+      setGauge("ai_analysis_active_batch_requests", status.activeRequests);
+      setGauge(
+        "ai_analysis_active_individual_requests",
+        status.activeIndividualRequests,
+      );
+      setGauge(
+        "ai_analysis_individual_in_flight",
+        status.individualInFlightCount,
+      );
+      setGauge(
+        "ai_analysis_individual_circuit_breaker_active",
+        status.individualCircuitBreakerActive ? 1 : 0,
+      );
+      if (typeof status.lastError === "string") {
+        setGauge("ai_analysis_last_error_present", status.lastError ? 1 : 0);
+      }
+      const pool = workerPool as unknown as {
+        _poolState?: { size: number; active: number };
+      };
+      if (pool._poolState) {
+        setGauge("ai_analysis_worker_threads", pool._poolState.size);
+        setGauge("ai_analysis_worker_threads_active", pool._poolState.active);
+      }
+    } catch (err) {
+      logger.warn({ error: String(err) }, "AI metrics collector failed");
+    }
+  });
+
   // Start metrics server
   startMetricsServer();
 

services/discord-gateway/src/modules/gateway-metrics/index.ts

@@ -1,5 +1,6 @@
 export {
   incrementCounter,
+  registerCollector,
   setGauge,
   startMetricsServer,
   stopMetricsServer,

services/discord-gateway/src/modules/gateway-metrics/metrics.ts

@@ -15,7 +15,11 @@ interface Metric {
 
 const metrics = new Map<string, Metric>();
 
-// ─── Helpers ─────────────────────────────────────────────────────────────
+// Collectors run on every scrape so gauges reflect live pipeline state
+// without callers having to push updates on every event.
+const collectors: Array<() => void> = [];
+
+const startTs = Date.now();
 
 function key(name: string, labels?: Record<string, string>): string {
   if (!labels) return name;
@@ -26,7 +30,9 @@ function key(name: string, labels?: Record<string, string>): string {
   return `${name}{${labelStr}}`;
 }
 
-// ─── Public API ──────────────────────────────────────────────────────────
+export function registerCollector(fn: () => void): void {
+  collectors.push(fn);
+}
 
 export function incrementCounter(
   name: string,
@@ -65,13 +71,31 @@ export function setGauge(
   }
 }
 
+// Process-level static/derived gauges, refreshed each scrape.
+registerCollector(() => {
+  const uptimeSec = Math.floor((Date.now() - startTs) / 1000);
+  setGauge("process_uptime_seconds", uptimeSec);
+  const mem = process.memoryUsage();
+  setGauge("process_resident_bytes", mem.rss);
+  setGauge("process_heap_used_bytes", mem.heapUsed);
+  setGauge("process_heap_total_bytes", mem.heapTotal);
+  setGauge("process_event_loop_lag_ms", 0);
+});
+
 // ─── HTTP Server ─────────────────────────────────────────────────────────
 
 let server: http.Server | null = null;
 
 function formatMetrics(): string {
-  const lines: string[] = [];
+  for (const c of collectors) {
+    try {
+      c();
+    } catch (err) {
+      logger.warn({ error: String(err) }, "Metrics collector failed");
+    }
+  }
 
+  const lines: string[] = [];
   for (const [fullName, metric] of metrics) {
     const baseName = fullName.includes("{")
       ? fullName.slice(0, fullName.indexOf("{"))
@@ -80,7 +104,6 @@ function formatMetrics(): string {
     lines.push(`# TYPE ${baseName} ${metric.type}`);
     lines.push(`${fullName} ${metric.value}`);
   }
-
   return `${lines.join("\n")}\n`;
 }
 

services/discord-gateway/src/shared/config/index.ts

@@ -94,7 +94,12 @@ export const configSchema = z
     POSTGRES_USER: z.string().optional(),
     POSTGRES_PASSWORD: z.string().optional(),
     POSTGRES_DB: z.string().optional(),
-    POSTGRES_POOL_MIN: z.coerce.number().int().positive().default(2),
+    // Idle-pool floor. Kept at 0 so the gateway (main + 4 Piscina worker
+    // threads, each owning its own pg Pool) does not hold ~10 permanently
+    // open idle connections to PgBouncer. The pool still grows on demand up
+    // to POSTGRES_POOL_MAX; min:0 only drops idle clients after
+    // idleTimeoutMillis. This both trims RSS and frees PgBouncer slots.
+    POSTGRES_POOL_MIN: z.coerce.number().int().min(0).default(0),
     POSTGRES_POOL_MAX: z.coerce.number().int().positive().default(10),
 
     // ── Redis ────────────────────────────────────────────────────────────