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>

src/server.py

@@ -445,6 +445,14 @@ async def forward_to_poke(
             async with httpx.AsyncClient(timeout=30) as http:
                 resp = await http.post(webhook_url, json=payload, headers=headers)
                 resp.raise_for_status()
+                # A 2xx status doesn't guarantee delivery — Poke can return
+                # HTTP 200 with {"success": false, ...} on a soft failure, so
+                # the status code alone isn't sufficient confirmation.
+                result = resp.json()
+                if not result.get("success", True):
+                    raise RuntimeError(
+                        f"Poke reported failure: {result.get('message', 'unknown error')}"
+                    )
                 logger.info(
                     "Forwarded email '%s' to Poke (status %d)",
                     email_data["subject"],