Update existing SimpleFIN connections on re-connect instead of creating duplicates

When a user re-connects SimpleFIN (e.g. after a key change), match existing
connections by institution name and update the access token in-place rather
than always creating new SyncConnection records. New linked accounts are
added if discovered, existing mappings are preserved.

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

src/Purrse.Api/Services/BankSyncService.cs

@@ -191,10 +191,47 @@ public class BankSyncService : IBankSyncService
         var accounts = await _simpleFin.GetAccountsAsync(config);
         var encryptedAccessUrl = _encryption.Encrypt(accessUrl);
 
+        // Load existing SimpleFIN connections for this user to avoid duplicates on re-connect
+        var existingConnections = await _db.SyncConnections
+            .Include(c => c.LinkedAccounts)
+            .Where(c => c.UserId == userId && c.Provider == SyncProvider.SimpleFIN)
+            .ToListAsync();
+
         var groupedByInstitution = accounts.GroupBy(a => a.Institution ?? "SimpleFIN Connection");
         var connectionIds = new List<Guid>();
 
         foreach (var group in groupedByInstitution)
+        {
+            // Check if a connection already exists for this institution
+            var existing = existingConnections.FirstOrDefault(c =>
+                c.InstitutionName == group.Key);
+
+            if (existing != null)
+            {
+                // Update the existing connection with the new access token
+                existing.EncryptedAccessToken = encryptedAccessUrl;
+                existing.IsActive = true;
+                existing.UpdatedAt = DateTime.UtcNow;
+                connectionIds.Add(existing.Id);
+
+                // Add any new linked accounts not already tracked
+                foreach (var account in group)
+                {
+                    if (!existing.LinkedAccounts.Any(la => la.ExternalAccountId == account.ExternalId))
+                    {
+                        _db.LinkedAccounts.Add(new LinkedAccount
+                        {
+                            Id = Guid.NewGuid(),
+                            SyncConnectionId = existing.Id,
+                            ExternalAccountId = account.ExternalId,
+                            ExternalAccountName = account.Name,
+                            ExternalAccountType = account.AccountType,
+                            LastKnownBalance = account.Balance
+                        });
+                    }
+                }
+            }
+            else
             {
                 var connection = new SyncConnection
                 {
@@ -221,6 +258,7 @@ public class BankSyncService : IBankSyncService
                     });
                 }
             }
+        }
 
         await _db.SaveChangesAsync();