feat: --commit flag (#44)

- --commit: после генерации отчёта сохраняет период в
  period.last_used.from/to YAML-конфига
- При period.dynamic=true следующий запуск вычисляет период от last_used:
  полный месяц → следующий месяц, произвольный диапазон → та же длина
- При period.dynamic=false перезаписывает default_from/default_to
- При period.precision=datetime сохраняет timestamps с точностью до секунд
- --commit без --output сохраняет отчёт по шаблону из конфига
- compute_next_period() в config.py
- save_period_to_config() в yaml_config.py
- 23 новых теста (7 CLI, 6 config, 6 yaml, 4 date_range)

Closes #44

tests/test_config.py

@@ -451,3 +451,109 @@ class TestConfigPeriodPrecision:
             cfg = AppConfig.from_yaml(yaml_path)
             assert cfg.period_last_used_from == ""
             assert cfg.period_last_used_to == ""
+
+
+class TestComputeNextPeriod:
+    """Tests for compute_next_period()."""
+
+    def test_full_month_goes_to_next_full_month(self):
+        from redmine_reporter.config import compute_next_period
+
+        nf, nt = compute_next_period("2026-06-01", "2026-06-30", "date")
+        assert nf == "2026-07-01"
+        assert nt == "2026-07-31"
+
+    def test_december_to_next_year_january(self):
+        from redmine_reporter.config import compute_next_period
+
+        nf, nt = compute_next_period("2025-12-01", "2025-12-31", "date")
+        assert nf == "2026-01-01"
+        assert nt == "2026-01-31"
+
+    def test_february_2026_non_leap_to_march(self):
+        from redmine_reporter.config import compute_next_period
+
+        nf, nt = compute_next_period("2026-02-01", "2026-02-28", "date")
+        assert nf == "2026-03-01"
+        assert nt == "2026-03-31"
+
+    def test_arbitrary_range_same_length_from_to_plus_one(self):
+        from redmine_reporter.config import compute_next_period
+
+        nf, nt = compute_next_period("2026-06-15", "2026-06-20", "date")
+        assert nf == "2026-06-21"
+        assert nt == "2026-06-26"
+
+    def test_single_day_range_moves_one_day(self):
+        from redmine_reporter.config import compute_next_period
+
+        nf, nt = compute_next_period("2026-06-15", "2026-06-15", "date")
+        assert nf == "2026-06-16"
+        assert nt == "2026-06-16"
+
+    def test_cross_month_range(self):
+        from redmine_reporter.config import compute_next_period
+
+        nf, nt = compute_next_period("2026-06-25", "2026-07-05", "date")
+        assert nf == "2026-07-06"
+        assert nt == "2026-07-16"
+
+    def test_datetime_precision_moves_by_seconds(self):
+        from redmine_reporter.config import compute_next_period
+
+        nf, nt = compute_next_period("2026-06-30T09:00:00", "2026-06-30T12:00:00", "datetime")
+        assert nf == "2026-06-30T12:00:01"
+        assert nt == "2026-06-30T15:00:01"
+
+
+class TestDefaultDateRangeWithLastUsed:
+    """Tests that get_default_date_range() uses last_used when dynamic=True."""
+
+    @mock.patch.dict(os.environ, {}, clear=True)
+    def test_uses_last_used_when_dynamic_true(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            yaml_path = Path(tmp) / "config.yml"
+            yaml_path.write_text(
+                "period:\n"
+                "  precision: date\n"
+                "  dynamic: true\n"
+                "  last_used:\n"
+                "    from: '2026-05-01'\n"
+                "    to: '2026-05-31'\n"
+            )
+            Config._app = AppConfig.from_yaml(yaml_path)
+            result = Config.get_default_date_range()
+            assert result == "2026-06-01--2026-06-30"
+
+    @mock.patch.dict(os.environ, {}, clear=True)
+    def test_ignores_last_used_when_dynamic_false(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            yaml_path = Path(tmp) / "config.yml"
+            yaml_path.write_text(
+                "period:\n"
+                "  precision: date\n"
+                "  dynamic: false\n"
+                "  default_from: '2026-03-01'\n"
+                "  default_to: '2026-03-15'\n"
+                "  last_used:\n"
+                "    from: '2026-06-01'\n"
+                "    to: '2026-06-30'\n"
+            )
+            Config._app = AppConfig.from_yaml(yaml_path)
+            result = Config.get_default_date_range()
+            assert result == "2026-03-01--2026-03-15"
+
+    @mock.patch.dict(os.environ, {}, clear=True)
+    def test_falls_back_when_no_last_used_even_with_dynamic(self):
+        with tempfile.TemporaryDirectory() as tmp:
+            yaml_path = Path(tmp) / "config.yml"
+            yaml_path.write_text(
+                "period:\n"
+                "  precision: date\n"
+                "  dynamic: true\n"
+                "  default_from: '2026-04-01'\n"
+                "  default_to: '2026-04-15'\n"
+            )
+            Config._app = AppConfig.from_yaml(yaml_path)
+            result = Config.get_default_date_range()
+            assert result == "2026-04-01--2026-04-15"