Make Resend from-address config-only; drop credential status rows

The Settings page no longer renders the Resend From Address input or any
of the read-only "Set in config.toml" credential status rows (Apify key,
eBay id/secret, ntfy auth, Resend key). Those rows looked like editable
inputs but were dead text, and duplicated what the Test buttons verify
more authoritatively.

- resend_from is now sourced only from config.toml ([resend] from); it is
  removed from settingsKeys, the test-Resend handler, the scheduler email
  client, and the schema seed.
- Removed the credStatus templ component, the CredentialStatus field on
  SettingsData, and the credentialStatus() handler helper.

internal/scheduler/email.go

@@ -21,11 +21,7 @@ func (s *Scheduler) emailClient(ctx context.Context) *email.Client {
 	if v, _ := s.store.GetSetting(ctx, "resend_api_key"); v != "" {
 		apiKey = v
 	}
-	from := s.cfg.Resend.From
-	if v, _ := s.store.GetSetting(ctx, "resend_from"); v != "" {
-		from = v
-	}
-	return email.New(apiKey, from)
+	return email.New(apiKey, s.cfg.Resend.From)
 }
 
 // dealMailer resolves the item owner and Resend client for deal-alert email.