Comment posting is idempotent + tests

.sqlx/query-3124db53d9e1fe0701a2fc70eea98e001fef4b75c24d33d8dd595f6b483e8f65.json

@@ -0,0 +1,14 @@
+{
+  "db_name": "PostgreSQL",
+  "query": "\n        INSERT INTO idempotency (idempotency_key, created_at)\n        VALUES ($1, now())\n        ON CONFLICT DO NOTHING\n        ",
+  "describe": {
+    "columns": [],
+    "parameters": {
+      "Left": [
+        "Text"
+      ]
+    },
+    "nullable": []
+  },
+  "hash": "3124db53d9e1fe0701a2fc70eea98e001fef4b75c24d33d8dd595f6b483e8f65"
+}