Fix Unspecified DateTime kind for PostgreSQL timestamptz columns

DateOnly.ToDateTime() produces Kind=Unspecified which Npgsql rejects
for timestamptz. Specify UTC kind on Plaid transaction dates and add
a safety net in BankSyncService for all parsed dates.

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

src/Purrse.Api/Services/BankSyncService.cs

@@ -442,7 +442,7 @@ public class BankSyncService : IBankSyncService
                     {
                         Id = Guid.NewGuid(),
                         AccountId = linkedAccount.AccountId!.Value,
-                        Date = parsed.Date,
+                        Date = DateTime.SpecifyKind(parsed.Date, DateTimeKind.Utc),
                         Amount = parsed.Amount,
                         PayeeId = matchedPayee?.Id,
                         PayeeName = parsed.PayeeName,

src/Purrse.Plugins.BankSync/PlaidSyncProvider.cs

@@ -168,7 +168,7 @@ public class PlaidSyncProvider : IAccountSyncProvider
             Success = true,
             Transactions = allTransactions.Select(t => new ParsedTransaction
             {
-                Date = t.Date?.ToDateTime(TimeOnly.MinValue) ?? DateTime.UtcNow,
+                Date = DateTime.SpecifyKind(t.Date?.ToDateTime(TimeOnly.MinValue) ?? DateTime.UtcNow, DateTimeKind.Utc),
                 Amount = (decimal)(t.Amount ?? 0) * -1, // Plaid uses positive for debits
                 PayeeName = t.MerchantName ?? t.Name,
                 Memo = t.Name,