Second checkpoint

internal/domain/errors.go

@@ -0,0 +1,16 @@
+// Package domain holds the core entities, enums, and sentinel errors shared by
+// every layer. It has no dependencies on other internal packages.
+package domain
+
+import "errors"
+
+var (
+	// ErrNotFound is returned when a requested entity does not exist.
+	ErrNotFound = errors.New("not found")
+	// ErrConflict is returned when an operation violates a constraint, e.g. a
+	// foreign-key reference or a uniqueness rule.
+	ErrConflict = errors.New("conflict")
+	// ErrValidation is returned when input fails a business rule (as opposed to
+	// request-shape validation, which the transport layer handles).
+	ErrValidation = errors.New("validation failed")
+)