Fix shared layer structure and DynamoDB Decimal type error (#2)

Move shared layer source from src/shared/python/shared/ to
src/shared/shared/ to prevent SAM from creating a double
python/python/ directory in the layer artifact. Add _to_decimal()
helper to convert floats to Decimal for DynamoDB compatibility
in put_order.

src/shared/shared/db.py

@@ -1,6 +1,8 @@
+import json
 import os
 import time
 from datetime import datetime
+from decimal import Decimal
 from zoneinfo import ZoneInfo
 from boto3.dynamodb.conditions import Key
 import boto3
@@ -63,8 +65,13 @@ def get_form_status(week: str) -> str:
     return menu.get("form_status", "closed")
 
 
+def _to_decimal(obj):
+    """Convert floats to Decimal for DynamoDB compatibility."""
+    return json.loads(json.dumps(obj), parse_float=Decimal)
+
+
 def put_order(week: str, employee_slug: str, order_data: dict):
-    _get_table().put_item(Item={
+    _get_table().put_item(Item=_to_decimal({
         "PK": f"WEEK#{week}",
         "SK": f"ORDER#{employee_slug}",
         "employee_name": order_data["employee_name"],
@@ -73,7 +80,7 @@ def put_order(week: str, employee_slug: str, order_data: dict):
         "items": order_data["items"],
         "total": order_data["total"],
         "ttl": ttl_days(90),
-    })
+    }))
 
 
 def get_orders(week: str) -> list[dict]: