PR validation workflow with CI services

[alekspetrov]

Parent: GH-36

Create .github/workflows/ci.yml and .env.ci. This is the most complex piece: 4 parallel jobs (Lint with golangci-lint, Test with race detector + coverage using PostgreSQL 16 and Redis 7 service containers, Security with gosec + govulncheck in report-only mode, Build verifying both go build and Docker build). Include Go module caching, coverage report as PR comment, and the .env.ci file with CI-specific database/Redis connection strings and test configuration. Triggered on PRs to main.

[alekspetrov]

✅ Pilot completed!

Metric	Value
Duration	7m46s
Branch	pilot/GH-63
PR	Warning: 1 uncommitted change
#80