test(tasks): improve test_api_tasks.py structure and add api marker

- Move admin_client, v9_client, user_client fixtures to conftest.py so
  they can be reused by other API tests; all three now build on the
  rest_api_client fixture instead of creating APIClient() directly
- Move regular_user fixture to conftest.py (was already done, now also
  used by the new client fixtures)
- Add docstrings to every test method describing the behaviour under test
- Move timedelta/timezone imports to module level
- Register 'api' pytest marker in pyproject.toml and apply pytestmark to
  the entire file so all 40 tests are selectable via -m api

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

pyproject.toml

@@ -311,6 +311,7 @@ markers = [
   "date_parsing: Tests which cover date parsing from content or filename",
   "management: Tests which cover management commands/functionality",
   "search: Tests for the Tantivy search backend",
+  "api: Tests for REST API endpoints",
 ]
 
 [tool.pytest_env]