refactor: consolidate settings lookups and secrets loading

- Extract fetch_setting/1 in Settings (4 callsites → 1 repo lookup)
- Replace hardcoded load_stripe_config with registry-driven load_all
- Adding new secrets is now a one-line @secret_registry entry
- Mark DRY refactor plan as complete (all 8 items done)

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

lib/simpleshop_theme/stripe/setup.ex

@@ -36,13 +36,13 @@ defmodule SimpleshopTheme.Stripe.Setup do
 
       case maybe_create_webhook(api_key) do
         {:ok, result} ->
-          Secrets.load_stripe_config()
+          Secrets.load_all()
           {:ok, result}
 
         {:error, reason} ->
           # Key is valid and stored, but webhook creation failed.
           # Still load the key so checkout works (webhooks can be set up manually).
-          Secrets.load_stripe_config()
+          Secrets.load_all()
           {:error, reason}
       end
     end
@@ -68,7 +68,7 @@ defmodule SimpleshopTheme.Stripe.Setup do
   """
   def save_signing_secret(signing_secret) do
     Settings.put_secret("stripe_signing_secret", signing_secret)
-    Secrets.load_stripe_config()
+    Secrets.load_all()
   end
 
   @doc """