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/tests/imageResizer.test.ts

@@ -1,5 +1,5 @@
-import { describe, expect, it } from "vitest";
 import sharp from "sharp";
+import { describe, expect, it } from "vitest";
 import { resizeImageForVision } from "../src/modules/attachment-upload/imageResizer.js";
 
 // Build a worst-case (poorly-compressing) 1024x1024 image, like a real photo.