diplomail (Stage D): language detection + lazy translation cache

Replaces the LangUndetermined placeholder with whatlanggo-backed
body detection on every send path, then adds a translation cache
keyed on (message_id, target_lang) populated lazily on the
per-message read endpoint. The noop translator that ships with
Stage D returns engine="noop", which the service treats as
"translation unavailable" — wiring a real backend (LibreTranslate
HTTP client is the documented next step) is a one-file swap.

GetMessage and ListInbox now accept a targetLang argument; the HTTP
layer resolves the caller's accounts.preferred_language and
forwards it. Inbox uses the cache only (never calls the
translator) so bulk reads stay fast under future SaaS backends.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/internal/diplomail/detector/detector_test.go

@@ -0,0 +1,49 @@
+package detector
+
+import "testing"
+
+func TestDetectKnownLanguages(t *testing.T) {
+	t.Parallel()
+	d := New()
+	cases := []struct {
+		name string
+		text string
+		want string
+	}{
+		{
+			name: "english paragraph",
+			text: "The trade agreement should be signed before the next turn. " +
+				"I expect a written response by the time the engine generates the next report.",
+			want: "en",
+		},
+		{
+			name: "russian paragraph",
+			text: "Привет! Я предлагаю заключить дипломатическое соглашение и провести " +
+				"совместную операцию по освоению гиперпространственных маршрутов. " +
+				"Жду твоего письменного ответа до конца следующего хода игры, " +
+				"чтобы мы успели согласовать детали и подписать договор вовремя.",
+			want: "ru",
+		},
+	}
+	for _, tc := range cases {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			got := d.Detect(tc.text)
+			if got != tc.want {
+				t.Fatalf("Detect = %q, want %q", got, tc.want)
+			}
+		})
+	}
+}
+
+func TestDetectShortOrEmpty(t *testing.T) {
+	t.Parallel()
+	d := New()
+	short := []string{"", "hi", "    "}
+	for _, s := range short {
+		if got := d.Detect(s); got != Undetermined {
+			t.Errorf("Detect(%q) = %q, want %q", s, got, Undetermined)
+		}
+	}
+}