fix: add multi-stage JSON parsing with fallback extraction

backend/app/services/ai/claude_client.py

@@ -58,11 +58,48 @@ def _extract_text(message) -> str:
 
 
 def _parse_json(text: str) -> dict:
+    # Strip markdown fences
     text = re.sub(r"^```(?:json)?\s*", "", text, flags=re.MULTILINE)
     text = re.sub(r"\s*```$", "", text, flags=re.MULTILINE)
     text = text.strip()
+
+    # First attempt: standard parse
     try:
         return json.loads(text)
+    except json.JSONDecodeError:
+        pass
+
+    # Second attempt: try with json5/demjson-style fixes
+    # Fix unescaped apostrophes inside JSON strings heuristically
+    # Replace 's that appear mid-word inside a string context
+    fixed = re.sub(r"(?<=[a-zA-Z])'(?=[a-zA-Z])", "\\'", text)
+    try:
+        return json.loads(fixed)
+    except json.JSONDecodeError:
+        pass
+
+    # Third attempt: extract just the cards array if top-level parse fails
+    # This handles truncated responses
+    cards_match = re.search(r'"cards"\s*:\s*(\[.*?\])\s*[,}]', text, re.DOTALL)
+    decklist_match = re.search(r'"decklist"\s*:\s*(\[.*?\])\s*[,}]', text, re.DOTALL)
+    deck_match = re.search(r'"deck"\s*:\s*(\[.*?\])\s*[,}]', text, re.DOTALL)
+
+    cards_text = None
+    for match in [cards_match, decklist_match, deck_match]:
+        if match:
+            cards_text = match.group(1)
+            break
+
+    if cards_text:
+        try:
+            cards = json.loads(cards_text)
+            return {"deck_name": "Generated Deck", "strategy_summary": "", "cards": cards}
+        except json.JSONDecodeError:
+            pass
+
+    # Last resort: raise with original error
+    try:
+        json.loads(text)
     except json.JSONDecodeError as e:
         raise ValueError(f"Claude response was not valid JSON: {e}\n\nRaw: {text[:500]}")