Gate POST /api/chat on the same step-up as /tools (#317)

/api/chat reaches the router, the LLM and, through applyAction, the whole
act path, so it is the widest state-changing surface mavweb serves. It was
the only one with no gate. It now goes through stepUpOK like POST /tools,
POST /routines and POST /api/revert: unchanged in the default deploy
(WebAuthn unconfigured, fail-open behind wg+nginx), 403 under
-require-stepup or an unasserted passkey session.

The route table now carries an explicit enumeration of every state-changing
route and its gate, and the two startup SECURITY log lines name /routines
and /api/chat alongside /tools and /api/revert.

The loopback -addr default the task also asked for landed earlier in
d12de58; the compose already publishes mavweb on 127.0.0.1 only.

cmd/mavweb/handlers_test.go

@@ -63,6 +63,18 @@ type fakeCore struct {
 	// for handleTrace tests
 	tickTrace ipc.TickTrace
 	traceErr  error
+
+	// for handleChatAPI tests
+	chatText string
+	chatErr  error
+}
+
+func (f *fakeCore) Chat(_ context.Context, text string) (string, error) {
+	f.chatText = text
+	if f.chatErr != nil {
+		return "", f.chatErr
+	}
+	return "поняла", nil
 }
 
 func (f *fakeCore) EnableTool(_ context.Context, name string, cmd []string, destructive bool, scope string, _ time.Time) error {
@@ -1045,3 +1057,64 @@ func TestHandleRoutines_NilCore_503(t *testing.T) {
 		t.Fatalf("status = %d, want 503", rr.Code)
 	}
 }
+
+// --- handleChatAPI step-up gate (Vikunja #317) ---
+//
+// POST /api/chat reaches the router, the LLM and the act path, so it carries
+// the same gate as POST /tools and POST /api/revert.
+
+func postChat(text string) *http.Request {
+	req := httptest.NewRequest(http.MethodPost, "/api/chat", strings.NewReader("text="+url.QueryEscape(text)))
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	return req
+}
+
+func TestHandleChatAPI_RequireStepUp_FailsClosed(t *testing.T) {
+	core := &fakeCore{}
+	rr := httptest.NewRecorder()
+	handleChatAPI(rr, postChat("выключи свет"), core, nil, true)
+	if rr.Code != http.StatusForbidden {
+		t.Fatalf("status = %d, want 403; body=%s", rr.Code, rr.Body.String())
+	}
+	if core.chatText != "" {
+		t.Errorf("core.Chat called with %q, but -require-stepup should deny", core.chatText)
+	}
+}
+
+func TestHandleChatAPI_UnassertedSession_Denied(t *testing.T) {
+	core := &fakeCore{}
+	rr := httptest.NewRecorder()
+	handleChatAPI(rr, postChat("выключи свет"), core, webauthn.NewPasskeySession(5*time.Minute), false)
+	if rr.Code != http.StatusForbidden {
+		t.Fatalf("status = %d, want 403", rr.Code)
+	}
+	if core.chatText != "" {
+		t.Errorf("core.Chat called with %q despite an unasserted session", core.chatText)
+	}
+}
+
+func TestHandleChatAPI_AssertedSession_PassesGate(t *testing.T) {
+	core := &fakeCore{}
+	rr := httptest.NewRecorder()
+	handleChatAPI(rr, postChat("привет"), core, stepUpSession(), true)
+	if rr.Code != http.StatusSeeOther {
+		t.Fatalf("status = %d, want 303; body=%s", rr.Code, rr.Body.String())
+	}
+	if core.chatText != "привет" {
+		t.Errorf("core.Chat text = %q, want %q", core.chatText, "привет")
+	}
+}
+
+// Default deploy: WebAuthn unconfigured and -require-stepup off ⇒ chat keeps
+// working, resting on the transport-level auth in front of mavweb.
+func TestHandleChatAPI_FailOpenByDefault(t *testing.T) {
+	core := &fakeCore{}
+	rr := httptest.NewRecorder()
+	handleChatAPI(rr, postChat("привет"), core, nil, false)
+	if rr.Code != http.StatusSeeOther {
+		t.Fatalf("status = %d, want 303", rr.Code)
+	}
+	if core.chatText != "привет" {
+		t.Errorf("core.Chat text = %q, want %q", core.chatText, "привет")
+	}
+}