add backup run contract

contracts/v1/backup-run-response.json

@@ -0,0 +1,11 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "type": "object",
+  "additionalProperties": false,
+  "required": ["status", "filename", "reason"],
+  "properties": {
+    "status": { "type": "string", "enum": ["ok", "failed"] },
+    "filename": { "type": ["string", "null"] },
+    "reason": { "type": ["string", "null"] }
+  }
+}

fixtures/v1/backup-run.response.json

@@ -0,0 +1,5 @@
+{
+  "status": "ok",
+  "filename": "trixie-20261025T030000Z.dump",
+  "reason": null
+}