Auto-populate loan details from Plaid liabilities and pre-fill setup form

Fetch loan data (mortgage/student) from Plaid's Liabilities API during sync
and auto-create LoanDetail records. For accounts without liabilities data,
pre-populate the setup form with balance, interest rate, and smart term
defaults. Also fixes maturity date computation in the frontend form.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/Purrse.Api/Services/LoanService.cs

@@ -1,5 +1,6 @@
 using Microsoft.EntityFrameworkCore;
 using Purrse.Core.DTOs;
+using Purrse.Core.Enums;
 using Purrse.Core.Interfaces.Services;
 using Purrse.Core.Models;
 using Purrse.Data;
@@ -154,6 +155,28 @@ public class LoanService : ILoanService
             totalInterestOriginal - totalInterestNew, monthsSaved, newEntries);
     }
 
+    public async Task<LoanDefaultsResponse> GetLoanDefaultsAsync(Guid userId, Guid accountId)
+    {
+        var account = await _db.Accounts
+            .Include(a => a.LinkedAccounts)
+            .FirstOrDefaultAsync(a => a.Id == accountId && a.UserId == userId)
+            ?? throw new KeyNotFoundException("Account not found");
+
+        var originalBalance = Math.Abs(account.Balance);
+        var interestRate = account.InterestRate ?? 0;
+        var hasSyncData = account.LinkedAccounts.Any() && (account.InterestRate.HasValue || account.Balance != 0);
+
+        var termMonths = account.Type switch
+        {
+            AccountType.Mortgage => 360,
+            AccountType.AutoLoan => 60,
+            AccountType.PersonalLoan => 60,
+            _ => 360
+        };
+
+        return new LoanDefaultsResponse(originalBalance, interestRate, termMonths, hasSyncData);
+    }
+
     private static List<AmortizationEntry> GenerateAmortizationSchedule(LoanDetail loan)
     {
         var entries = new List<AmortizationEntry>();