feat: add activity feed contract

contracts/v1/activity-list-response.json

@@ -0,0 +1,18 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://trixie.reaperofveriod.nl/contracts/v1/activity-list-response.json",
+  "title": "Trixie activity feed response",
+  "type": "array",
+  "items": {
+    "type": "object",
+    "additionalProperties": false,
+    "required": ["event_id", "event_type", "occurred_at", "payload"],
+    "properties": {
+      "event_id": { "type": "string", "minLength": 1 },
+      "event_type": { "type": "string", "pattern": "^[a-z0-9_.-]+$" },
+      "occurred_at": { "type": "string", "format": "date-time" },
+      "payload": { "type": "object" }
+    }
+  },
+  "maxItems": 100
+}

fixtures/v1/activity-list.response.json

@@ -0,0 +1,8 @@
+[
+  {
+    "event_id": "synthetic-event-1",
+    "event_type": "channel.follow",
+    "occurred_at": "2026-10-25T12:00:00Z",
+    "payload": { "user_name": "SyntheticViewer" }
+  }
+]