fix: convert Decimal to float for JSON serialization in evaluation

- PostgreSQL returns numeric values as Decimal objects
- psycopg2.Json() cannot serialize Decimal to JSON
- Added convert_decimals() helper function
- Converts activity_data, context, and evaluation_result before saving

Fixes: Batch evaluation errors (31 errors 'Decimal is not JSON serializable')

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/evaluation_helper.py

@@ -6,6 +6,7 @@ Issue: #15
 Date: 2026-03-23
 """
 from typing import Dict, Optional, List
+from decimal import Decimal
 import logging
 
 from db import get_cursor
@@ -14,6 +15,21 @@ from profile_evaluator import TrainingProfileEvaluator
 logger = logging.getLogger(__name__)
 
 
+def convert_decimals(obj):
+    """
+    Recursively converts Decimal objects to float for JSON serialization.
+
+    PostgreSQL returns numeric values as Decimal, but psycopg2.Json() can't serialize them.
+    """
+    if isinstance(obj, Decimal):
+        return float(obj)
+    elif isinstance(obj, dict):
+        return {k: convert_decimals(v) for k, v in obj.items()}
+    elif isinstance(obj, list):
+        return [convert_decimals(item) for item in obj]
+    return obj
+
+
 def load_parameters_registry(cur) -> Dict[str, Dict]:
     """
     Loads training parameters registry from database.
@@ -145,17 +161,24 @@ def evaluate_and_save_activity(
         lookback_days=30
     )
 
+    # Convert Decimal values in activity_data and context
+    activity_data_clean = convert_decimals(activity_data)
+    context_clean = convert_decimals(context)
+
     # Evaluate
     evaluator = TrainingProfileEvaluator(parameters)
     evaluation_result = evaluator.evaluate_activity(
-        activity_data,
+        activity_data_clean,
         profile,
-        context
+        context_clean
     )
 
     # Save to database
     from psycopg2.extras import Json
 
+    # Convert Decimal to float for JSON serialization
+    evaluation_result_clean = convert_decimals(evaluation_result)
+
     cur.execute("""
         UPDATE activity_log
         SET evaluation = %s,
@@ -163,9 +186,9 @@ def evaluate_and_save_activity(
             overall_score = %s
         WHERE id = %s
     """, (
-        Json(evaluation_result),
-        evaluation_result.get("quality_label"),
-        evaluation_result.get("overall_score"),
+        Json(evaluation_result_clean),
+        evaluation_result_clean.get("quality_label"),
+        evaluation_result_clean.get("overall_score"),
         activity_id
     ))