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/diplomail_e2e_test.go

@@ -808,6 +808,36 @@ func TestDiplomailLifecycleMembershipKick(t *testing.T) {
 	}
 }
 
+// TestDiplomailWorkerTickOnEmptyQueueIsNoop confirms the async
+// worker tolerates an empty pending queue: no error, no panic, no
+// publisher events. Belt-and-suspenders for the case where backend
+// starts, mounts the worker as an `app.Component`, and ticks before
+// any user has sent mail.
+func TestDiplomailWorkerTickOnEmptyQueueIsNoop(t *testing.T) {
+	db := startPostgres(t)
+	ctx := context.Background()
+
+	publisher := &recordingPublisher{}
+	svc := diplomail.NewService(diplomail.Deps{
+		Store:        diplomail.NewStore(db),
+		Memberships:  &staticMembershipLookup{},
+		Notification: publisher,
+		Config: config.DiplomailConfig{
+			MaxBodyBytes:    4096,
+			MaxSubjectBytes: 256,
+		},
+	})
+	worker := diplomail.NewWorker(svc)
+	for i := 0; i < 3; i++ {
+		if err := worker.Tick(ctx); err != nil {
+			t.Fatalf("tick %d on empty queue: %v", i, err)
+		}
+	}
+	if got := publisher.snapshot(); len(got) != 0 {
+		t.Fatalf("publisher fired %d events on empty queue", len(got))
+	}
+}
+
 // TestDiplomailAsyncTranslationDelivery covers the Stage E flow:
 //  1. SendPersonal where recipient.preferred_language != body_lang
 //     materialises a recipient with `AvailableAt == nil`; the inbox