add collaborator contract

contracts/v1/collaborators-response.json

@@ -0,0 +1,15 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "array",
+  "items": {
+    "type": "object",
+    "additionalProperties": false,
+    "required": ["twitch_user_id", "display_name", "favorite", "last_invited_at"],
+    "properties": {
+      "twitch_user_id": { "type": "string" },
+      "display_name": { "type": "string" },
+      "favorite": { "type": "boolean" },
+      "last_invited_at": { "type": ["string", "null"] }
+    }
+  }
+}

fixtures/v1/collaborators.response.json

@@ -0,0 +1,8 @@
+[
+  {
+    "twitch_user_id": "123456",
+    "display_name": "ExampleStreamer",
+    "favorite": true,
+    "last_invited_at": "2026-10-25T12:00:00Z"
+  }
+]