fix: make JSON structure explicit in system prompt

backend/app/services/ai/prompts.py

@@ -26,7 +26,17 @@ SYSTEM_PROMPT = (
     "You always respond with a single valid JSON object — no markdown fences, no "
     "preamble, no commentary outside the JSON. Your card names must exactly match "
     "official Magic card names (English). Every card must be legal in Commander "
-    "and within the commander's colour identity."
+    "and within the commander's colour identity.\n\n"
+    "CRITICAL: Your response must use EXACTLY this JSON structure:\n"
+    "{\n"
+    '  "deck_name": "string",\n'
+    '  "strategy_summary": "string",\n'
+    '  "cards": [\n'
+    '    {"name": "Card Name", "slot": "creature|instant|sorcery|enchantment|artifact|planeswalker|land|battle", "quantity": 1, "reasoning": "..."}\n'
+    "  ],\n"
+    '  "cuts": []\n'
+    "}\n"
+    "Do NOT use any other key names. The cards array must contain exactly the non-commander cards."
 )