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/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 ────────────────────────────────────────────────────────────