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/Controllers/LoansController.cs

@@ -23,6 +23,10 @@ public class LoansController : ControllerBase
         return detail == null ? NotFound() : Ok(detail);
     }
 
+    [HttpGet("{accountId}/defaults")]
+    public async Task<ActionResult<LoanDefaultsResponse>> GetLoanDefaults(Guid accountId)
+        => Ok(await _loanService.GetLoanDefaultsAsync(UserId, accountId));
+
     [HttpPost("{accountId}")]
     public async Task<ActionResult<LoanDetailResponse>> CreateLoanDetail(Guid accountId, CreateLoanDetailRequest request)
         => Ok(await _loanService.CreateLoanDetailAsync(UserId, accountId, request));

src/Purrse.Api/Services/BankSyncService.cs

@@ -21,6 +21,7 @@ public class BankSyncService : IBankSyncService
     private readonly IDuplicateDetectionService _duplicateService;
     private readonly IPayeeService _payeeService;
     private readonly IAiCategorizationService _aiCategorization;
+    private readonly ILoanService _loanService;
     private readonly IHubContext<NotificationHub> _hub;
     private readonly ILogger<BankSyncService> _logger;
 
@@ -32,6 +33,7 @@ public class BankSyncService : IBankSyncService
         IDuplicateDetectionService duplicateService,
         IPayeeService payeeService,
         IAiCategorizationService aiCategorization,
+        ILoanService loanService,
         IHubContext<NotificationHub> hub,
         ILogger<BankSyncService> logger)
     {
@@ -42,6 +44,7 @@ public class BankSyncService : IBankSyncService
         _duplicateService = duplicateService;
         _payeeService = payeeService;
         _aiCategorization = aiCategorization;
+        _loanService = loanService;
         _hub = hub;
         _logger = logger;
     }
@@ -413,6 +416,12 @@ public class BankSyncService : IBankSyncService
         var accountResults = new List<SyncAccountResult>();
         int totalFetched = 0, totalImported = 0, totalSkipped = 0;
 
+        // Try to auto-create loan details from Plaid liabilities data
+        if (connection.Provider == SyncProvider.Plaid)
+        {
+            await TryCreateLoanDetailsFromPlaidAsync(userId, connection, config);
+        }
+
         // For SimpleFIN, use the earliest start date across all accounts so the
         // single API call (cached) returns transactions covering every account.
         var endDate = DateTime.UtcNow;
@@ -625,6 +634,77 @@ public class BankSyncService : IBankSyncService
             accountResults);
     }
 
+    private async Task TryCreateLoanDetailsFromPlaidAsync(Guid userId, SyncConnection connection, Dictionary<string, string> config)
+    {
+        try
+        {
+            var liabilities = await _plaid.GetLiabilitiesAsync(config);
+            if (liabilities.Count == 0) return;
+
+            var linkedAccounts = connection.LinkedAccounts
+                .Where(la => la.IsEnabled && la.AccountId != null)
+                .ToList();
+
+            foreach (var liability in liabilities)
+            {
+                var linked = linkedAccounts.FirstOrDefault(la => la.ExternalAccountId == liability.ExternalAccountId);
+                if (linked?.AccountId == null) continue;
+
+                var accountId = linked.AccountId.Value;
+
+                // Skip if loan detail already exists
+                var existing = await _loanService.GetLoanDetailAsync(userId, accountId);
+                if (existing != null) continue;
+
+                // Update account interest rate from liabilities data
+                if (liability.InterestRate.HasValue && linked.Account != null)
+                {
+                    linked.Account.InterestRate = liability.InterestRate.Value;
+                }
+
+                var originalBalance = liability.OriginalBalance ?? Math.Abs(linked.LastKnownBalance ?? 0);
+                var interestRate = liability.InterestRate ?? 0;
+                var termMonths = liability.TermMonths ?? (liability.LoanType == "Mortgage" ? 360 : 60);
+                var originationDate = liability.OriginationDate ?? DateTime.UtcNow;
+                var escrowBalance = liability.EscrowBalance;
+
+                // Compute monthly payment via amortization formula if not provided
+                var monthlyPayment = liability.MonthlyPayment ?? ComputeMonthlyPayment(originalBalance, interestRate, termMonths);
+
+                var maturityDate = liability.MaturityDate ?? originationDate.AddMonths(termMonths);
+
+                var request = new CreateLoanDetailRequest(
+                    originalBalance,
+                    interestRate,
+                    termMonths,
+                    monthlyPayment,
+                    DateTime.SpecifyKind(originationDate, DateTimeKind.Utc),
+                    DateTime.SpecifyKind(maturityDate, DateTimeKind.Utc),
+                    escrowBalance,
+                    null
+                );
+
+                await _loanService.CreateLoanDetailAsync(userId, accountId, request);
+                _logger.LogInformation("Auto-created loan details for account {AccountId} from Plaid liabilities", accountId);
+            }
+        }
+        catch (Exception ex)
+        {
+            _logger.LogWarning(ex, "Failed to fetch Plaid liabilities for connection {ConnectionId} - sync will continue without loan details", connection.Id);
+        }
+    }
+
+    private static decimal ComputeMonthlyPayment(decimal principal, decimal annualRatePercent, int termMonths)
+    {
+        if (principal <= 0 || termMonths <= 0) return 0;
+        if (annualRatePercent <= 0) return principal / termMonths;
+
+        var monthlyRate = annualRatePercent / 100m / 12m;
+        var factor = (double)monthlyRate;
+        var pow = Math.Pow(1 + factor, termMonths);
+        return principal * (decimal)(factor * pow / (pow - 1));
+    }
+
     private static Dictionary<string, string> BuildProviderConfig(SyncConnection connection, string accessToken, PlaidConfig? plaidCredentials)
     {
         if (connection.Provider == SyncProvider.Plaid && plaidCredentials != null)

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>();