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/config/config.go

@@ -43,9 +43,9 @@ func (c AuthConfig) ForwardAuthEnabled() bool {
 	return strings.EqualFold(strings.TrimSpace(c.Mode), "forward")
 }
 
-// ResendConfig holds Resend transactional-email credentials. APIKey and From
-// can both be overridden at runtime via /settings (resend_api_key,
-// resend_from). From is an RFC-5322 address, e.g. "Veola <veola@example.com>".
+// ResendConfig holds Resend transactional-email credentials, set only in
+// config.toml (never via the web UI). From is an RFC-5322 address, e.g.
+// "Veola <veola@example.com>".
 type ResendConfig struct {
 	APIKey string `toml:"api_key"`
 	From   string `toml:"from"`