🚀 Modernize: Update Dependencies, Architecture & CI

[drzombey]

Summary

Complete modernization of the project after 2 years of inactivity.

Phase 1 — Dependencies & Deprecated APIs

• Go: 1.21 → 1.24.0
• Docker SDK: v24 → v28.5
• OpenTelemetry: Deprecated Jaeger exporter → OTLP HTTP exporter
• Docker API types: Migrated to new sub-packages (container.*, image.*, registry.*)
• All dependencies updated to latest versions

Phase 2 — Architecture Refactoring

• Dependency Injection: Replaced global handler variables with Handler struct
• DB Connection Pool: Single initialization at startup instead of per-request
• Error Middleware: Structured error types with Gin panic recovery middleware
• Graceful Shutdown: Signal handling (SIGINT/SIGTERM) with 30s timeout
• 12-Factor Config: AUR_BUILDER_ env prefix via Viper for environment variable support

Phase 3 — Tests, README, CI/CD

• Tests: 8 new handler unit tests with mocked dependencies + PackageRepository interface
• README: Complete rewrite with architecture overview, API docs, badges, deployment guide
• CI: All GitHub Actions updated (checkout@v4, setup-go@v5), golangci-lint integrated
• Dependabot: Auto-updates for Go modules, Actions, and Docker images
• Dockerfile: Updated to Go 1.24-alpine

Verification

• ✅ go build ./... — compiles cleanly
• ✅ go vet ./... — no issues
• ✅ go test ./... — all tests passing
• ✅ No breaking API changes