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):

tests/test_client.py

@@ -1,6 +1,8 @@
 import os
 from unittest import mock
 
+import pytest
+
 from redmine_reporter.client import fetch_issues_with_spent_time
 from redmine_reporter.config import DEFAULT_REDMINE_VERIFY
 
@@ -168,6 +170,85 @@ def test_fetch_uses_custom_verify_path(mock_redmine_class):
     assert kwargs["requests"]["verify"] == "/tmp/redmine-ca.pem"
 
 
+@mock.patch.dict(os.environ, PASSWORD_ENV, clear=True)
+@mock.patch("redmine_reporter.client.Redmine")
+def test_fetch_raises_redmine_api_error_on_auth(mock_redmine_class):
+    """AuthError превращается в RedmineAPIError с понятным сообщением."""
+    from redminelib.exceptions import AuthError
+
+    mock_redmine = mock_redmine_class.return_value
+    mock_redmine.user.get.side_effect = AuthError()
+
+    from redmine_reporter.client import RedmineAPIError
+
+    with pytest.raises(RedmineAPIError, match="Authentication failed"):
+        fetch_issues_with_spent_time("2026-01-01", "2026-01-31")
+
+
+@mock.patch.dict(os.environ, PASSWORD_ENV, clear=True)
+@mock.patch("redmine_reporter.client.Redmine")
+def test_fetch_raises_redmine_api_error_on_forbidden(mock_redmine_class):
+    """ForbiddenError превращается в RedmineAPIError с понятным сообщением."""
+    from redminelib.exceptions import ForbiddenError
+
+    mock_redmine = mock_redmine_class.return_value
+    mock_redmine.user.get.side_effect = ForbiddenError()
+
+    from redmine_reporter.client import RedmineAPIError
+
+    with pytest.raises(RedmineAPIError, match="Access denied"):
+        fetch_issues_with_spent_time("2026-01-01", "2026-01-31")
+
+
+@mock.patch.dict(os.environ, PASSWORD_ENV, clear=True)
+@mock.patch("redmine_reporter.client.Redmine")
+def test_fetch_raises_redmine_api_error_on_timeout(mock_redmine_class):
+    """requests Timeout превращается в RedmineAPIError с понятным сообщением."""
+    import requests
+
+    mock_redmine = mock_redmine_class.return_value
+    mock_redmine.user.get.side_effect = requests.exceptions.Timeout("timeout")
+
+    from redmine_reporter.client import RedmineAPIError
+
+    with pytest.raises(RedmineAPIError, match="timed out"):
+        fetch_issues_with_spent_time("2026-01-01", "2026-01-31")
+
+
+@mock.patch.dict(os.environ, PASSWORD_ENV, clear=True)
+@mock.patch("redmine_reporter.client.Redmine")
+def test_fetch_raises_redmine_api_error_on_connection_error(mock_redmine_class):
+    """ConnectionError превращается в RedmineAPIError с понятным сообщением."""
+    import requests
+
+    mock_redmine = mock_redmine_class.return_value
+    mock_redmine.user.get.side_effect = requests.exceptions.ConnectionError("no route")
+
+    from redmine_reporter.client import RedmineAPIError
+
+    with pytest.raises(RedmineAPIError, match="Cannot connect"):
+        fetch_issues_with_spent_time("2026-01-01", "2026-01-31")
+
+
+@mock.patch.dict(os.environ, PASSWORD_ENV, clear=True)
+@mock.patch("redmine_reporter.client.Redmine")
+def test_fetch_raises_redmine_api_error_on_http_500(mock_redmine_class):
+    """HTTP 500 превращается в RedmineAPIError с понятным сообщением."""
+    import requests
+
+    mock_redmine = mock_redmine_class.return_value
+    response = requests.Response()
+    response.status_code = 500
+    mock_redmine.user.get.side_effect = requests.exceptions.HTTPError(
+        "server error", response=response
+    )
+
+    from redmine_reporter.client import RedmineAPIError
+
+    with pytest.raises(RedmineAPIError, match="server error"):
+        fetch_issues_with_spent_time("2026-01-01", "2026-01-31")
+
+
 # -- #24: Таймаут и retry --