feat(1099): add payer EIN to ChurchProfile (entity, DTO, migration)

Add PayerEin (nullable string, max 20) to ChurchProfile entity, AppDbContext
config, ChurchProfileDto response, UpdateChurchProfileRequest, and
ChurchProfileService round-trip — mirroring the Phone/BankRoutingNumber
nullable-string pattern. Migration AddPayerEinToChurchProfile adds only the
one nullable column to ChurchProfiles.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

API/ROLAC.API/Migrations/AppDbContextModelSnapshot.cs

@@ -506,6 +506,10 @@ namespace ROLAC.API.Migrations
                     b.Property<int>("NextCheckNumber")
                         .HasColumnType("integer");
 
+                    b.Property<string>("PayerEin")
+                        .HasMaxLength(20)
+                        .HasColumnType("character varying(20)");
+
                     b.Property<string>("Phone")
                         .HasMaxLength(50)
                         .HasColumnType("character varying(50)");