add overlay response contract

contracts/v1/overlay-response.json

@@ -0,0 +1,16 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://trixie.reaperofveriod.nl/contracts/v1/overlay-response.json",
+  "title": "Trixie public overlay",
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["slug", "html", "css", "javascript", "enabled", "disabled_design"],
+  "properties": {
+    "slug": { "type": "string", "pattern": "^[a-z0-9-]+$" },
+    "html": { "type": "string" },
+    "css": { "type": "string" },
+    "javascript": { "type": "string" },
+    "enabled": { "type": "boolean" },
+    "disabled_design": { "type": "string" }
+  }
+}

fixtures/v1/overlay.response.json

@@ -0,0 +1,8 @@
+{
+  "slug": "followers",
+  "html": "<div id=\"alert\"></div>",
+  "css": "#alert { color: white; }",
+  "javascript": "",
+  "enabled": true,
+  "disabled_design": "transparent"
+}