Make change tracking retention configurable, defaulting to 30 days

src/Data/DataStore.cs

@@ -59,11 +59,29 @@ public class SqliteDataStore : DataStore
 
 public class SqlServerDataStore : DataStore
 {
+    /// <summary>
+    /// The default number of days SQL Server Change Tracking keeps change history for.
+    /// </summary>
+    /// <remarks>
+    /// A client that stays offline longer than this window cannot resume an incremental sync and has
+    /// to be reinitialized from a fresh snapshot, so the window has to comfortably cover how long a
+    /// device is expected to go without connectivity. Longer retention grows the change tracking side
+    /// tables, which is the cost being traded against here.
+    /// </remarks>
+    public const int DefaultChangeRetentionDays = 30;
+
     public required string ConnectionString { get; set; }
 
     public string GetResolvedConnectionString() => ResolveConnectionString(ConnectionString);
 
     public SqlServerDataStoreTrackingMode TrackingMode { get; set; }
+
+    /// <summary>
+    /// How many days of change history to retain, applied when
+    /// <see cref="TrackingMode"/> is <see cref="SqlServerDataStoreTrackingMode.ChangeTracking"/>.
+    /// Ignored for the trigger-based tracking mode, which keeps its own journal.
+    /// </summary>
+    public int ChangeRetentionDays { get; set; } = DefaultChangeRetentionDays;
 }
 
 public class PostgreSqlDataStore : DataStore