Fix Poke webhook: verify success field, point key setup to Kitchen

- forward_to_poke now checks the response's success field instead of
  trusting the HTTP status alone; Poke can return 200 with
  success: false on a soft failure, which was previously logged and
  treated as delivered.
- README/start.sh pointed users to Settings > Advanced for the API
  key, which issues a legacy pk_ key incompatible with the
  inbound/api-message endpoint this project uses. Now points to
  poke.com/kitchen -> API Keys for a V2 key.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

README.md

@@ -61,11 +61,11 @@ Set up poke-mail (https://github.com/kacperkwapisz/poke-mail) for me — clone t
 cp config.example.yml config.yml
 ```
 
-Edit `config.yml` with your email credentials and Poke API key (from [poke.com/settings/advanced](https://poke.com/settings/advanced)):
+Edit `config.yml` with your email credentials and Poke API key (from [poke.com/kitchen](https://poke.com/kitchen) → API Keys — **not** Settings → Advanced or the Recipes page; those issue a legacy `pk_` key that only works with the deprecated `inbound-sms/webhook` endpoint, not the one below):
 
 ```yaml
 webhook_url: https://poke.com/api/v1/inbound/api-message
-poke_api_key: your-api-key  # from https://poke.com/settings/advanced
+poke_api_key: your-api-key  # from https://poke.com/kitchen → API Keys (V2 key required for this endpoint)
 
 accounts:
   # iCloud Mail — login is @icloud.com, send as your custom domain