Documentation - Service diagrams

[dieperid]

Summary

This PR adds project documentation diagrams to make the metrics service easier to understand from both a system and runtime perspective.

What Changed

• Added an updated architecture/data-flow diagram in documentation/architecture/dataFlowDiagram.md
• Added sequence diagrams for the main request flows in documentation/architecture/sequenceDiagram.md
• Added a deployment diagram showing the Docker Compose runtime setup in documentation/architecture/deploymentDiagram.md
• Updated README.md to link to the new architecture documentation

Details

The architecture/data-flow diagram was updated to preserve the upstream ingestion context with the temperature sensor, MQTT broker, ingestion worker, and device registry service, while also showing how requests are handled inside this repository through the Gin router, handlers, metric registry/validator, VictoriaMetrics service, and VictoriaMetrics itself.

The sequence diagrams document the main application flows:

• metric ingestion via POST /api/v1/metrics
• sensor status retrieval via GET /api/v1/sensors/:sensorId/status

The deployment diagram was added because the service behavior depends on the Docker Compose boundary between the Go API container and the VictoriaMetrics container, including environment-based configuration and persistent storage.

Notes

This PR is documentation-only and does not change application behavior.