Add comprehensive Matrix alerting system with Grafana integration

- Implement custom Python alerting system (src/alerting.py) with water level monitoring, data freshness checks, and Matrix notifications
- Add complete Grafana Matrix alerting setup guide (docs/GRAFANA_MATRIX_SETUP.md) with webhook configuration, alert rules, and notification policies
- Create Matrix quick start guide (docs/MATRIX_QUICK_START.md) for rapid deployment
- Integrate alerting commands into main application (--alert-check, --alert-test)
- Add Matrix configuration to environment variables (.env.example)
- Update Makefile with alerting targets (alert-check, alert-test)
- Enhance status command to show Matrix notification status
- Support station-specific water level thresholds and escalation rules
- Provide dual alerting approach: native Grafana alerts and custom Python system

🤖 Generated with [Claude Code](https://claude.ai/code)

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

Makefile

@@ -21,6 +21,11 @@ help:
 	@echo "  run          Run the monitor in continuous mode"
 	@echo "  run-api      Run the web API server"
 	@echo "  run-test     Run a single test cycle"
+	@echo "  run-status   Show system status"
+	@echo ""
+	@echo "Alerting:"
+	@echo "  alert-check  Check water levels and send alerts"
+	@echo "  alert-test   Send test Matrix message"
 	@echo ""
 	@echo "Distribution:"
 	@echo "  build-exe    Build standalone executable"
@@ -92,6 +97,13 @@ run-test:
 run-status:
 	uv run python run.py --status
 
+# Alerting
+alert-check:
+	uv run python run.py --alert-check
+
+alert-test:
+	uv run python run.py --alert-test
+
 # Docker
 docker-build:
 	docker build -t ping-river-monitor .