[MM-64360] Data retention: optionally preserve pinned posts (#31165)

* add new config to preserve pinned posts during data retention

* more graceful error if the pinned post was a reply in a deleted thread

server/public/model/config.go

@@ -3119,6 +3119,7 @@ type DataRetentionSettings struct {
 	BatchSize                      *int    `access:"compliance_data_retention_policy"`
 	TimeBetweenBatchesMilliseconds *int    `access:"compliance_data_retention_policy"`
 	RetentionIdsBatchSize          *int    `access:"compliance_data_retention_policy"`
+	PreservePinnedPosts            *bool   `access:"compliance_data_retention_policy"`
 }
 
 func (s *DataRetentionSettings) SetDefaults() {
@@ -3168,6 +3169,10 @@ func (s *DataRetentionSettings) SetDefaults() {
 	if s.RetentionIdsBatchSize == nil {
 		s.RetentionIdsBatchSize = NewPointer(DataRetentionSettingsDefaultRetentionIdsBatchSize)
 	}
+
+	if s.PreservePinnedPosts == nil {
+		s.PreservePinnedPosts = NewPointer(false)
+	}
 }
 
 // GetMessageRetentionHours returns the message retention time as an int.

server/public/model/data_retention_policy.go

@@ -91,6 +91,13 @@ type RetentionIdsForDeletion struct {
 	Ids       []string
 }
 
+type RetentionPolicyBatchConfigs struct {
+	Now                 int64
+	GlobalPolicyEndTime int64
+	Limit               int64
+	PreservePinnedPosts bool
+}
+
 func (r *RetentionIdsForDeletion) PreSave() {
 	if r.Id == "" {
 		r.Id = NewId()