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/Entities/ChurchProfile.cs

@@ -21,6 +21,9 @@ public class ChurchProfile : AuditableEntity, IAuditable
     public string? BankAccountNumber { get; set; }
     public string? BankRoutingNumber { get; set; }
 
+    /// <summary>Payer EIN printed on Form 1099-NEC Copy B; the church's own public business identifier.</summary>
+    public string? PayerEin          { get; set; }
+
     // ── AI assist provider settings (editable via Church Profile → AI 設定 tab) ──
     public string  AiProvider   { get; set; } = "Claude";   // "Claude" | "Gemini"
     public string? ClaudeModel  { get; set; } = "claude-haiku-4-5-20251001";