feat(config): add config-as-code YAML import/export

Add declarative config-as-code support via YAML files. Monitors and
alerts can be exported, version controlled, and applied across instances.

- goupkeep export [-o file.yaml] dumps current state
- goupkeep apply -f file.yaml creates/updates to match desired state
- --dry-run shows planned changes without applying
- --prune deletes monitors/alerts not in the YAML
- Matching by name, alert references by name, nested group children
- CLI refactored to subcommands (apply, export, serve) with backward compat
- 24 tests covering apply, export, validation, round-trip idempotency

internal/metrics/prometheus_test.go

@@ -34,8 +34,16 @@ func (m *mockStore) SaveCheck(int, int64, bool) error                         {
 func (m *mockStore) LoadAllHistory(int) (map[int][]models.CheckRecord, error) {
 	return nil, nil
 }
-func (m *mockStore) ExportData() (models.Backup, error) { return models.Backup{}, nil }
-func (m *mockStore) ImportData(models.Backup) error     { return nil }
+func (m *mockStore) ExportData() (models.Backup, error)        { return models.Backup{}, nil }
+func (m *mockStore) ImportData(models.Backup) error            { return nil }
+func (m *mockStore) GetSiteByName(string) (models.Site, error) { return models.Site{}, nil }
+func (m *mockStore) GetAlertByName(string) (models.AlertConfig, error) {
+	return models.AlertConfig{}, nil
+}
+func (m *mockStore) AddSiteReturningID(models.Site) (int, error) { return 0, nil }
+func (m *mockStore) AddAlertReturningID(string, string, map[string]string) (int, error) {
+	return 0, nil
+}
 
 func TestMetricsHandler(t *testing.T) {
 	ms := &mockStore{