feat: add POST /pulls/{owner}/{repo}/{index}/assignees route

Register the PR assignees route pointing to the existing AssignIssue
handler, which works for both issues and PRs via the Gitea API.
Add integration tests covering valid, HTMX, and missing assignee cases.

Closes leeworks-agents/gitea-mobile#228

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/handlers/handlers.go

@@ -56,6 +56,7 @@ func (h *Handler) RegisterRoutes(mux *http.ServeMux) {
 	mux.HandleFunc("GET /pulls/{owner}/{repo}/{index}", h.PullDetail)
 	mux.HandleFunc("POST /pulls/{owner}/{repo}/{index}/review", h.SubmitReview)
 	mux.HandleFunc("POST /pulls/{owner}/{repo}/{index}/state", h.SetPullState)
+	mux.HandleFunc("POST /pulls/{owner}/{repo}/{index}/assignees", h.AssignIssue)
 
 	// Settings (handled separately for auth bypass).
 	settingsHandler := &SettingsHandler{

internal/handlers/integration_test.go

@@ -726,6 +726,66 @@ func TestIntegration_AssignIssue_MissingAssignee(t *testing.T) {
 	}
 }
 
+// --- Issue #228: Integration tests for POST /pulls/{owner}/{repo}/{index}/assignees ---
+
+func TestIntegration_AssignPull_Valid(t *testing.T) {
+	h, srv := newTestHandlerWithMock(t)
+	defer srv.Close()
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("POST /pulls/{owner}/{repo}/{index}/assignees", h.AssignIssue)
+
+	form := url.Values{"assignee": {"user1"}}
+	req := reqWithToken(http.MethodPost, "/pulls/test-org/repo1/1/assignees", form.Encode())
+	w := httptest.NewRecorder()
+
+	mux.ServeHTTP(w, req)
+
+	if w.Code != http.StatusSeeOther {
+		t.Errorf("status = %d, want %d", w.Code, http.StatusSeeOther)
+	}
+}
+
+func TestIntegration_AssignPull_HTMX(t *testing.T) {
+	h, srv := newTestHandlerWithMock(t)
+	defer srv.Close()
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("POST /pulls/{owner}/{repo}/{index}/assignees", h.AssignIssue)
+
+	form := url.Values{"assignee": {"user1"}}
+	req := reqWithToken(http.MethodPost, "/pulls/test-org/repo1/1/assignees", form.Encode())
+	req.Header.Set("HX-Request", "true")
+	w := httptest.NewRecorder()
+
+	mux.ServeHTTP(w, req)
+
+	if w.Code != http.StatusOK {
+		t.Errorf("status = %d, want %d", w.Code, http.StatusOK)
+	}
+	body := w.Body.String()
+	if !contains(body, "Assigned to user1") {
+		t.Errorf("expected 'Assigned to user1' in HTMX response, got: %s", body)
+	}
+}
+
+func TestIntegration_AssignPull_MissingAssignee(t *testing.T) {
+	h, srv := newTestHandlerWithMock(t)
+	defer srv.Close()
+
+	mux := http.NewServeMux()
+	mux.HandleFunc("POST /pulls/{owner}/{repo}/{index}/assignees", h.AssignIssue)
+
+	req := reqWithToken(http.MethodPost, "/pulls/test-org/repo1/1/assignees", "")
+	w := httptest.NewRecorder()
+
+	mux.ServeHTTP(w, req)
+
+	if w.Code != http.StatusBadRequest {
+		t.Errorf("status = %d, want %d", w.Code, http.StatusBadRequest)
+	}
+}
+
 // --- Issue #118: Integration tests for CloseIssue and AddComment ---
 
 func TestIntegration_CloseIssue(t *testing.T) {