fix: migrate Todoist client from deprecated REST v2 to API v1

REST v2 (https://api.todoist.com/rest/v2) is being sunset — the /close
endpoint was returning HTTP 410 in production, breaking task completion.

Update base URL to https://api.todoist.com/api/v1; endpoint paths are
unchanged (/tasks/{id}/close, /tasks/{id}/reopen, etc.).

Test: TestTodoistClient_UsesAPIv1BaseURL asserts the correct base URL.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

2 files changed, 9 insertions, 1 deletions

diff --git a/internal/api/todoist.go b/internal/api/todoist.go
index 6068d2e..2745e3e 100644
--- a/internal/api/todoist.go
+++ b/internal/api/todoist.go
@@ -10,7 +10,7 @@ import (
 )
 
 const (
-	todoistBaseURL     = "https://api.todoist.com/rest/v2"
+	todoistBaseURL     = "https://api.todoist.com/api/v1"
 	todoistSyncBaseURL = "https://api.todoist.com/sync/v9"
 )
 
diff --git a/internal/api/todoist_test.go b/internal/api/todoist_test.go
index f7ca719..159be61 100644
--- a/internal/api/todoist_test.go
+++ b/internal/api/todoist_test.go
@@ -143,6 +143,14 @@ func TestTodoistClient_CreateTask_WithDueDate(t *testing.T) {
 	}
 }
 
+func TestTodoistClient_UsesAPIv1BaseURL(t *testing.T) {
+	client := NewTodoistClient("test-key")
+	const want = "https://api.todoist.com/api/v1"
+	if client.BaseURL != want {
+		t.Errorf("expected base URL %q, got %q — Todoist REST v2 is deprecated (HTTP 410)", want, client.BaseURL)
+	}
+}
+
 func TestTodoistClient_CompleteTask(t *testing.T) {
 	// Mock server
 	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {