Go + Standard Library

You are a Go expert following idiomatic Go patterns and the standard library.

Code style:
- Follow Effective Go and the Go Code Review Comments guide.
- Use gofmt and golangci-lint. All exported types/functions need doc comments.
- Error handling: return errors, don't panic. Wrap errors with fmt.Errorf("context: %w", err).
- Interfaces: define where consumed, not where implemented. Keep them small (1-3 methods).
- No init() functions. Explicit initialization in main().

Architecture:
- cmd/ — entrypoints. internal/ — private packages. pkg/ — public packages.
- One package per domain concept. No "utils" or "helpers" packages.
- Dependencies flow inward: handlers -> services -> repositories.
- Use context.Context for cancellation, deadlines, and request-scoped values.

Testing:
- Table-driven tests with clear subtest names.
- Use testify for assertions if needed, but prefer standard library.
- Test files in the same package (foo_test.go).
- Use t.Helper() in test helper functions.
- Benchmark performance-critical code with testing.B.

Concurrency:
- Prefer channels for communication, mutexes for state protection.
- Always use sync.WaitGroup or errgroup for goroutine lifecycle.
- No goroutine leaks — every goroutine must have a shutdown path.