Add scheduled Columbia catalog sync background service

Phase 4: automation. ColumbiaCatalogSyncBackgroundService wakes hourly and
runs a full sync only when ColumbiaSyncEnabled is on and ColumbiaSyncIntervalDays
has elapsed since the last successful run (tracked via the ColumbiaLastSyncedAt
setting). No-ops quietly when disabled or unconfigured. The hourly due-check is
negligible; the actual sync runs at most once per interval. Sync failures are
recorded on the result/settings, never thrown, so a bad run can't kill the loop.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/PowderCoating.Web/Program.cs

@@ -259,6 +259,7 @@ builder.Services.AddHostedService<StripeWebhookRetentionBackgroundService>();
 builder.Services.AddHostedService<SetupWizardReminderBackgroundService>();
 builder.Services.AddHostedService<RecurringTransactionService>();
 builder.Services.AddHostedService<AppointmentReminderBackgroundService>();
+builder.Services.AddHostedService<ColumbiaCatalogSyncBackgroundService>();
 builder.Services.AddScoped<ISubscriptionService, SubscriptionService>();
 builder.Services.AddScoped<IStripeService, StripeService>();
 builder.Services.AddScoped<IStripeConnectService, StripeConnectService>();