docs(deployment): update memory limits and celery concurrency across all guides

Sync all deployment docs with actual docker-compose.yml values:
- celery-worker: 512→768MB, concurrency 4→2
- db: 512→256MB, celery-beat: 256→128MB, flower: 256→192MB
- Redis maxmemory: 256mb→100mb (matches container mem_limit 128m)
- Add redis-exporter to scaling guide memory budget

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/deployment/hetzner-server-setup.md

@@ -161,7 +161,7 @@ Complete step-by-step guide for deploying Orion on a Hetzner Cloud VPS.
 !!! success "Progress — 2026-02-17"
     **Launch readiness — fully deployed and verified (44/44 checks pass):**
 
-    - **Memory limits** on all 6 app containers (db: 512m, redis: 128m, api: 512m, celery-worker: 512m, celery-beat: 256m, flower: 256m) — beat/flower bumped from 128m after OOM kills
+    - **Memory limits** on all 6 app containers (db: 256m, redis: 128m, api: 512m, celery-worker: 768m, celery-beat: 128m, flower: 192m) — rebalanced after celery-worker OOM kills (concurrency reduced from 4 to 2)
     - **Flower port** restricted to localhost only (`127.0.0.1:5555:5555`) — access via Caddy reverse proxy
     - **Flower password** changed from default
     - **Infrastructure health checks** — `/health/ready` now checks PostgreSQL (`SELECT 1`) and Redis (`ping`) with individual check details and latency