🚀 Improve robustness, docs, and deployment quality checks

[gepser]

Summary

This PR upgrades project quality and reliability across runtime behavior, tooling, and documentation.

✅ What changed

• Hardened HTTP handler behavior in progress.go
  • Input validation for percentage and color params
  • Percentage clamping (0..100)
  • GET/HEAD only (405 for unsupported methods)
  • Explicit Cache-Control and Content-Type
  • Safer error handling (no fatal exits in request path)
  • Request-level structured logs with latency
• Added test coverage in progress_test.go
  • Success path
  • Invalid percentage/color
  • Method validation
  • Clamp behavior
  • HEAD behavior
• Modernized local dev tooling
  • Added mise config (.mise.toml)
  • Added Makefile (setup, run, fmt, test, vet, check, tidy, smoke)
  • Updated local server runner in cmd/main.go
• Added deployment validation tools
  • scripts/smoke.sh for endpoint contract checks
  • GitHub workflow for manual smoke tests (.github/workflows/smoke.yml)
• Added maintenance automation
  • Dependabot config (.github/dependabot.yml)
• Improved docs
  • Expanded API contract and status codes in README.md
  • Added contributor onboarding guide (CONTRIBUTING.md)

🧪 Validation

• mise exec -- make fmt
• mise exec -- make check
• mise exec -- make tidy
• Local smoke test against http://localhost:8080

Issues

• Closes Add post-deploy smoke tests for the production endpoint #8
• Closes Improve production observability for HTTP requests #10
• Closes Expand project documentation and contributor onboarding #11
• Related to Set up dependency update automation (Dependabot + periodic checks) #9 (dependency update automation is included; full dependency upgrade cadence remains tracked there)