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/Services/ChurchProfileService.cs

@@ -18,7 +18,7 @@ public class ChurchProfileService : IChurchProfileService
             Id = p.Id, Name = p.Name, NameZh = p.NameZh, Phone = p.Phone, Email = p.Email,
             Website = p.Website, Address = p.Address, City = p.City, State = p.State,
             ZipCode = p.ZipCode, BankName = p.BankName, BankAccountNumber = p.BankAccountNumber,
-            BankRoutingNumber = p.BankRoutingNumber, NextCheckNumber = p.NextCheckNumber,
+            BankRoutingNumber = p.BankRoutingNumber, PayerEin = p.PayerEin, NextCheckNumber = p.NextCheckNumber,
             AiProvider = p.AiProvider,
             ClaudeModel = p.ClaudeModel,
             ClaudeApiKeyMasked = Mask(p.ClaudeApiKey),
@@ -33,7 +33,7 @@ public class ChurchProfileService : IChurchProfileService
         p.Name = r.Name; p.NameZh = r.NameZh; p.Phone = r.Phone; p.Email = r.Email;
         p.Website = r.Website; p.Address = r.Address; p.City = r.City; p.State = r.State;
         p.ZipCode = r.ZipCode; p.BankName = r.BankName; p.BankAccountNumber = r.BankAccountNumber;
-        p.BankRoutingNumber = r.BankRoutingNumber; p.NextCheckNumber = r.NextCheckNumber;
+        p.BankRoutingNumber = r.BankRoutingNumber; p.PayerEin = r.PayerEin; p.NextCheckNumber = r.NextCheckNumber;
         p.AiProvider  = string.IsNullOrWhiteSpace(r.AiProvider) ? "Claude" : r.AiProvider;
         p.ClaudeModel = r.ClaudeModel;
         p.GeminiModel = r.GeminiModel;