feat(postit): persist call IP privacy preference

Store HideIpAddressInCalls on user accounts, expose it in the profile API and PostIt editor, and preserve preferences for older clients. Add the EF migration, regression tests and architecture documentation.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/Yavsc.Abstract/Identity/Me.cs

@@ -5,6 +5,7 @@ namespace Yavsc.Abstract.Identity {
   {
       public bool EmailConfirmed { get; set; }
       public bool AllowMonthlyEmail { get; set; }
+      public bool HideIpAddressInCalls { get; set; }
       public bool HasPassword { get; set; }
       public bool TwoFactorEnabled { get; set; }
       public int ExternalLoginCount { get; set; }

src/Yavsc.Abstract/Identity/ProfileUpdateRequest.cs

@@ -9,4 +9,5 @@ public class ProfileUpdateRequest
     public string? Address { get; set; }
     public string? GoogleCalendarId { get; set; }
     public string? BankInfoSummary { get; set; }
+    public bool? HideIpAddressInCalls { get; set; }
 }