feat(monitoring): add Redis exporter + Sentry docs to deployment guide

- Add redis-exporter container to docker-compose (oliver006/redis_exporter, 32MB)
- Add Redis scrape target to Prometheus config
- Add 4 Redis alert rules: RedisDown, HighMemory, HighConnections, RejectedConnections
- Document Step 19b (Sentry Error Tracking) in Hetzner deployment guide
- Document Step 19c (Redis Monitoring) in Hetzner deployment guide
- Update resource budget and port reference tables

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

requirements-docs.txt

@@ -4,4 +4,4 @@ mkdocs>=1.6.1
 mkdocs-material>=9.5.48
 mkdocstrings[python]>=0.27.0
 mkdocs-swagger-ui-tag>=0.6.10
-pymdown-extensions>=10.12
+pymdown-extensions>=10.12