add backup inventory contract

contracts/v1/backup-list-response.json

@@ -0,0 +1,14 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "array",
+  "items": {
+    "type": "object",
+    "additionalProperties": false,
+    "required": ["filename", "size_bytes", "modified_at"],
+    "properties": {
+      "filename": { "type": "string" },
+      "size_bytes": { "type": "integer", "minimum": 0 },
+      "modified_at": { "type": ["string", "null"] }
+    }
+  }
+}

fixtures/v1/backup-list.response.json

@@ -0,0 +1,7 @@
+[
+  {
+    "filename": "trixie-20261025T030000Z.dump",
+    "size_bytes": 4096,
+    "modified_at": "2026-10-25T03:00:01Z"
+  }
+]