fix: Sanitize JSON response for NaN/Inf float values

Add sanitize_for_json() helper that recursively traverses response
and replaces NaN/Infinity values with null. Fixes JSON serialization
errors when attention/entropy calculations produce invalid floats.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[email protected]>

backend/model_service.py

@@ -27,6 +27,33 @@ from .architectural_analysis import extract_architectural_data
 logging.basicConfig(level=logging.INFO)
 logger = logging.getLogger(__name__)
 
+import math
+
+def sanitize_for_json(obj):
+    """
+    Recursively sanitize a nested data structure for JSON serialization.
+    Replaces NaN and Infinity float values with None (JSON null).
+    """
+    if isinstance(obj, dict):
+        return {k: sanitize_for_json(v) for k, v in obj.items()}
+    elif isinstance(obj, list):
+        return [sanitize_for_json(item) for item in obj]
+    elif isinstance(obj, float):
+        if math.isnan(obj) or math.isinf(obj):
+            return None  # JSON null
+        return obj
+    elif isinstance(obj, (np.floating, np.float32, np.float64)):
+        val = float(obj)
+        if math.isnan(val) or math.isinf(val):
+            return None
+        return val
+    elif isinstance(obj, (np.integer, np.int32, np.int64)):
+        return int(obj)
+    elif isinstance(obj, np.ndarray):
+        return sanitize_for_json(obj.tolist())
+    else:
+        return obj
+
 app = FastAPI(title="Visualisable.ai Model Service", version="0.1.0")
 
 # CORS configuration for local development and production
@@ -1779,7 +1806,8 @@ async def analyze_research_attention(request: Dict[str, Any], authenticated: boo
 
         logger.info(f"✅ Research attention analysis complete: {len(generated_tokens)} tokens, {generation_time:.2f}s")
 
-        return response
+        # Sanitize response to handle NaN/Inf values that break JSON serialization
+        return sanitize_for_json(response)
 
     except Exception as e:
         logger.error(f"Research attention analysis error: {e}")