feat: add zulip integration

remote/cache-service/README.md

@@ -42,3 +42,34 @@ NixOS options under `services.noisebell-cache`:
 | `piApiKeyFile` | Bearer token to poll the Pi's GET endpoints |
 | `inboundApiKeyFile` | Bearer token the Pi sends when POSTing to `/webhook` |
 | `outboundWebhooks[].secretFile` | Bearer token sent with outbound webhook POSTs |
+
+## Home Assistant
+
+Home Assistant works well as another `outboundWebhooks` target.
+
+- For fast automations, point a webhook target at Home Assistant's `/api/webhook/<webhook_id>` endpoint.
+- For current-state display and restart recovery, have Home Assistant also poll `GET /status`.
+
+Example NixOS config:
+
+```nix
+{
+  services.noisebell-cache.outboundWebhooks = [
+    {
+      url = "https://homeassistant.example.com/api/webhook/noisebell-door";
+      secretFile = null;
+    }
+  ];
+}
+```
+
+The webhook payload sent by cache-service is:
+
+```json
+{
+  "status": "open",
+  "timestamp": 1710000000
+}
+```
+
+`secretFile = null` is usually fine for Home Assistant webhook triggers because the webhook ID itself acts as the secret. If you place Home Assistant behind an extra auth layer that expects a bearer token, set `secretFile` to a file containing that token.