feat(domain): Language, a closed set carried to the engines on the context (#61)

sv and auto; an empty form value is sv, anything else is refused. LanguageFrom defaults to
sv so a caller that sets nothing behaves as before.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/domain/language.go

@@ -0,0 +1,47 @@
+package domain
+
+import "context"
+
+// Language is the spoken language an upload is transcribed as. It is chosen by the user on
+// the upload form and reaches the engines as one request field, so it is a closed set,
+// never free text (tapir#61).
+type Language string
+
+const (
+	// LangSwedish sends language=sv: the Swedish engines' own language, and what every
+	// upload was transcribed as before the choice existed.
+	LangSwedish Language = "sv"
+	// LangAuto sends no language, so the engine detects it. It is what "Other" on the form
+	// means: the Swedish-only engines are not used for it.
+	LangAuto Language = "auto"
+)
+
+// ParseLanguage reads a form value. An empty value is Swedish (a client that predates the
+// field); anything that is not exactly a known value is refused rather than normalised.
+func ParseLanguage(s string) (Language, bool) {
+	switch Language(s) {
+	case "", LangSwedish:
+		return LangSwedish, true
+	case LangAuto:
+		return LangAuto, true
+	}
+	return "", false
+}
+
+type languageKey struct{}
+
+// WithLanguage returns a context that carries the language of the job it serves. The
+// Transcriber interface does not change: engines read it with LanguageFrom, the same way
+// they read the progress Reporter (ADR-035).
+func WithLanguage(ctx context.Context, l Language) context.Context {
+	return context.WithValue(ctx, languageKey{}, l)
+}
+
+// LanguageFrom returns the language carried by ctx, or Swedish when it carries none, so a
+// caller that sets nothing behaves as it always did.
+func LanguageFrom(ctx context.Context) Language {
+	if l, ok := ctx.Value(languageKey{}).(Language); ok && l != "" {
+		return l
+	}
+	return LangSwedish
+}