chore: harden CI and docker-compose defaults

[matisku]

Title
Harden CI and docker-compose defaults

Summary

• Replace legacy CI with GitHub Actions, adding smoke + integration checks, security scanning, and SBOM output.
• Modernize docker-compose.yml (services + named volumes), add env-driven configuration, and remove unnecessary privileged/Docker socket access.
• Add .env.example, Makefile, and Renovate config to standardize local workflows and dependency updates.
• Update README with CI/branch-protection guidance, updated ports, and security notes.

Changes

• Add .github/workflows/ci.yml with compose build, readiness checks, InfluxDB write/read + schema validation, Trivy scans, and SBOM artifact.
• Add renovate.json for Docker image updates with digest pinning.
• Add .env.example and Makefile for consistent local usage.
• Update docker-compose.yml to use env defaults, named volumes, and safer defaults.
• Update README to reflect CI, ports, and security guidance.
• Remove .travis.yml.

Testing

• Not run locally (Docker unavailable). CI will validate on push/PR.