fix: resolve --user-login by exact match

Redmine user.filter(login=...) performs an inexact substring search, but
_resolve_user_id took users[0].id unconditionally, so a report could
silently be built for the wrong user (e.g. 'ivanov' matching 'ivanov2').

Now only exact (case-sensitive) login matches are considered: exactly one
match resolves to its id, multiple matches raise an ambiguity error,
no match falls through to the name lookup and then to a 'not found'
error — symmetric with the --user-name resolution.

Closes #60

tests/test_client.py

@@ -275,6 +275,7 @@ def test_fetch_resolves_user_login_to_id(mock_redmine_class):
 
     mock_user = mock.MagicMock()
     mock_user.id = 7
+    mock_user.login = "ivanov"
     mock_redmine.user.filter.return_value = [mock_user]
     mock_redmine.time_entry.filter.return_value = []
 
@@ -287,6 +288,91 @@ def test_fetch_resolves_user_login_to_id(mock_redmine_class):
     assert kwargs["user_id"] == 7
 
 
+@mock.patch.dict(os.environ, PASSWORD_ENV, clear=True)
+@mock.patch("redmine_reporter.client.Redmine")
+def test_fetch_rejects_inexact_login_match(mock_redmine_class):
+    """Фильтр Redmine по логину неточный (#60): без точного совпадения — ошибка."""
+    mock_redmine = mock_redmine_class.return_value
+    _configure_current_user(mock_redmine)
+
+    near_match = mock.MagicMock()
+    near_match.id = 8
+    near_match.login = "ivanov2"
+    mock_redmine.user.filter.side_effect = [
+        [near_match],  # неточное совпадение по логину
+        [],  # по имени никого не найдено
+    ]
+
+    from redmine_reporter.client import RedmineAPIError
+
+    with pytest.raises(RedmineAPIError, match="not found"):
+        fetch_issues_with_spent_time("2026-01-01", "2026-01-31", user_id="ivanov")
+
+
+@mock.patch.dict(os.environ, PASSWORD_ENV, clear=True)
+@mock.patch("redmine_reporter.client.Redmine")
+def test_fetch_resolves_exact_login_among_near_matches(mock_redmine_class):
+    """Среди неточных результатов фильтра выбирается точное совпадение логина (#60)."""
+    mock_redmine = mock_redmine_class.return_value
+    _configure_current_user(mock_redmine)
+
+    near = mock.MagicMock()
+    near.id = 8
+    near.login = "ivanov2"
+    exact = mock.MagicMock()
+    exact.id = 7
+    exact.login = "ivanov"
+    mock_redmine.user.filter.return_value = [near, exact]
+    mock_redmine.time_entry.filter.return_value = []
+
+    fetch_issues_with_spent_time("2026-01-01", "2026-01-31", user_id="ivanov")
+
+    _, kwargs = mock_redmine.time_entry.filter.call_args
+    assert kwargs["user_id"] == 7
+
+
+@mock.patch.dict(os.environ, PASSWORD_ENV, clear=True)
+@mock.patch("redmine_reporter.client.Redmine")
+def test_fetch_rejects_multiple_exact_login_matches(mock_redmine_class):
+    """Несколько пользователей с точным логином — ошибка неоднозначности (#60)."""
+    mock_redmine = mock_redmine_class.return_value
+    _configure_current_user(mock_redmine)
+
+    user1 = mock.MagicMock()
+    user1.id = 7
+    user1.login = "ivanov"
+    user2 = mock.MagicMock()
+    user2.id = 9
+    user2.login = "ivanov"
+    mock_redmine.user.filter.return_value = [user1, user2]
+
+    from redmine_reporter.client import RedmineAPIError
+
+    with pytest.raises(RedmineAPIError, match="Multiple users match"):
+        fetch_issues_with_spent_time("2026-01-01", "2026-01-31", user_id="ivanov")
+
+
+@mock.patch.dict(os.environ, PASSWORD_ENV, clear=True)
+@mock.patch("redmine_reporter.client.Redmine")
+def test_fetch_login_exact_match_is_case_sensitive(mock_redmine_class):
+    """Точное совпадение логина регистрозависимое (#60)."""
+    mock_redmine = mock_redmine_class.return_value
+    _configure_current_user(mock_redmine)
+
+    other_case = mock.MagicMock()
+    other_case.id = 8
+    other_case.login = "Ivanov"
+    mock_redmine.user.filter.side_effect = [
+        [other_case],  # логин отличается регистром
+        [],  # по имени никого не найдено
+    ]
+
+    from redmine_reporter.client import RedmineAPIError
+
+    with pytest.raises(RedmineAPIError, match="not found"):
+        fetch_issues_with_spent_time("2026-01-01", "2026-01-31", user_id="ivanov")
+
+
 @mock.patch.dict(os.environ, PASSWORD_ENV, clear=True)
 @mock.patch("redmine_reporter.client.Redmine")
 def test_fetch_resolves_user_name_to_id(mock_redmine_class):