fix(options): relax EmailAccountsOptions properties from init-only to set

init-only properties cannot be mutated by Options configure delegates.
Switching to set allows the builder pattern to bind values post-construction.
Also narrows Accounts type from IEnumerable to List for index access.

src/core/DigitalData.MessagingService.Application/Common/Options/EmailAccountsOptions.cs

@@ -17,18 +17,18 @@ public class EmailAccountsOptions
     /// <summary>
     /// The list of configured email accounts.
     /// </summary>
-    public required IEnumerable<EmailAccountModificationDto> Accounts { get; init; } = [];
+    public required List<EmailAccountModificationDto> Accounts { get; set; } = [];
 
     /// <summary>
     /// How often the IMAP sync worker polls for new emails, in seconds.
     /// Defaults to 300 seconds (5 minutes).
     /// </summary>
-    public int SyncIntervalSeconds { get; init; } = 300;
+    public int SyncIntervalSeconds { get; set; } = 300;
 
     /// <summary>
     /// The minimum interval, in seconds, between forced IMAP sync operations regardless of idle state.
     /// Defaults to 30 seconds (0.5 minute).
     /// </summary>
-    public int ForcedSyncIntervalSeconds { get; init; } = 30;
+    public int ForcedSyncIntervalSeconds { get; set; } = 30;
 }
 #endif