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.Org/Migrations/20261025155604_HideIpAddressInCalls.cs

@@ -0,0 +1,29 @@
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace Yavsc.Migrations
+{
+    /// <inheritdoc />
+    public partial class HideIpAddressInCalls : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.AddColumn<bool>(
+                name: "HideIpAddressInCalls",
+                table: "AspNetUsers",
+                type: "boolean",
+                nullable: false,
+                defaultValue: false);
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropColumn(
+                name: "HideIpAddressInCalls",
+                table: "AspNetUsers");
+        }
+    }
+}

src/Yavsc.Org/Migrations/ApplicationDbContextModelSnapshot.cs

@@ -1170,6 +1170,9 @@ namespace Yavsc.Migrations
                         .HasMaxLength(512)
                         .HasColumnType("character varying(512)");
 
+                    b.Property<bool>("HideIpAddressInCalls")
+                        .HasColumnType("boolean");
+
                     b.Property<bool>("LockoutEnabled")
                         .HasColumnType("boolean");