feat(errors): provide readable Redmine API error messages

- Introduce RedmineAPIError with human-friendly messages.
- Distinguish AuthError, ForbiddenError, HTTP status codes, timeouts
  and connection errors in client.py.
- Update CLI to print the readable message instead of generic
  "Redmine API error: ...".
- Log original exception with traceback when --debug is enabled.
- Add tests for all error paths and CLI output.

Closes #39

tests/test_cli.py

@@ -210,6 +210,46 @@ def test_total_issues_message_goes_to_stderr(mock_fetch, capsys):
     assert "Total issues" in captured.err
 
 
+@mock.patch.dict(os.environ, VALID_ENV, clear=True)
+@mock.patch("redmine_reporter.cli.fetch_issues_with_spent_time")
+def test_cli_prints_readable_auth_error(mock_fetch, capsys):
+    """CLI выводит понятное сообщение при ошибке аутентификации."""
+    from redmine_reporter.client import RedmineAPIError
+
+    mock_fetch.side_effect = RedmineAPIError("Authentication failed: bad key")
+    code = main(["--date", "2026-01-01--2026-01-31"])
+    captured = capsys.readouterr()
+    assert code == 1
+    assert "Authentication failed" in captured.err
+    assert "Redmine API error" not in captured.err
+
+
+@mock.patch.dict(os.environ, VALID_ENV, clear=True)
+@mock.patch("redmine_reporter.cli.fetch_issues_with_spent_time")
+def test_cli_prints_readable_forbidden_error(mock_fetch, capsys):
+    """CLI выводит понятное сообщение при недостаточных правах."""
+    from redmine_reporter.client import RedmineAPIError
+
+    mock_fetch.side_effect = RedmineAPIError("Access denied: no permission")
+    code = main(["--date", "2026-01-01--2026-01-31"])
+    captured = capsys.readouterr()
+    assert code == 1
+    assert "Access denied" in captured.err
+
+
+@mock.patch.dict(os.environ, VALID_ENV, clear=True)
+@mock.patch("redmine_reporter.cli.fetch_issues_with_spent_time")
+def test_cli_prints_readable_timeout_error(mock_fetch, capsys):
+    """CLI выводит понятное сообщение при таймауте."""
+    from redmine_reporter.client import RedmineAPIError
+
+    mock_fetch.side_effect = RedmineAPIError("Redmine request timed out after 30 seconds")
+    code = main(["--date", "2026-01-01--2026-01-31"])
+    captured = capsys.readouterr()
+    assert code == 1
+    assert "timed out" in captured.err
+
+
 @mock.patch.dict(os.environ, VALID_ENV, clear=True)
 @mock.patch("redmine_reporter.cli.fetch_issues_with_spent_time")
 def test_cli_no_time_passed_to_formatter(mock_fetch, tmp_path):