diplomail (Stage F): docs + edge-case tests + LibreTranslate recipe

Closes the documentation gaps from the freshly-audited diplomail
implementation. FUNCTIONAL.md gains a §11 "Diplomatic mail" with
the full user-facing story across all five stages, mirrored into
FUNCTIONAL_ru.md as the project conventions require. A new
backend/docs/diplomail-translator-setup.md captures the
LibreTranslate operational recipe (Docker image, env wiring,
manual smoke test, troubleshooting). The package README gains a
"Multi-instance posture" note documenting the deliberate absence
of FOR UPDATE in the worker pickup query — single-instance is
safe today; multi-instance scaling will revisit the claim
mechanism.

Two small edge-case tests round things out: malformed
LibreTranslate response bodies (single string, short array,
empty array, missing field) must surface as errors so the worker
falls back instead of crashing; and an empty translation queue
must produce zero events on three consecutive Worker.Tick calls.

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

backend/internal/diplomail/translator/libretranslate_test.go

@@ -139,3 +139,35 @@ func TestLibreTranslateRequiresURL(t *testing.T) {
 		t.Fatalf("expected error for empty URL")
 	}
 }
+
+// TestLibreTranslateRejectsMalformedArray defends against a server
+// that returns a partial / unexpected `translatedText` payload. The
+// client must surface an error (not panic, not return a half-empty
+// Result) so the worker can decide between retry and fallback.
+func TestLibreTranslateRejectsMalformedArray(t *testing.T) {
+	t.Parallel()
+	cases := []struct {
+		name string
+		body string
+	}{
+		{"single string", `{"translatedText": "only one"}`},
+		{"array of one", `{"translatedText": ["only one"]}`},
+		{"empty array", `{"translatedText": []}`},
+		{"missing field", `{"foo":"bar"}`},
+	}
+	for _, tc := range cases {
+		body := tc.body
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, _ *http.Request) {
+				_, _ = w.Write([]byte(body))
+			}))
+			t.Cleanup(server.Close)
+			tr, _ := NewLibreTranslate(LibreTranslateConfig{URL: server.URL})
+			res, err := tr.Translate(context.Background(), "en", "ru", "subject", "body")
+			if err == nil {
+				t.Fatalf("expected error for malformed body %q, got %+v", body, res)
+			}
+		})
+	}
+}