feat(logging): convert planner/routes.py nurture warning to logging

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

web/src/padelnomics/planner/routes.py

@@ -3,10 +3,13 @@ Planner domain: padel court financial planner + scenario management.
 """
 
 import json
+import logging
 import math
 from datetime import datetime
 from pathlib import Path
 
+logger = logging.getLogger(__name__)
+
 from quart import Blueprint, Response, g, jsonify, render_template, request
 
 from ..auth.routes import login_required
@@ -533,7 +536,7 @@ async def save_scenario():
                     }
                 )
             except Exception as e:
-                print(f"[NURTURE] Failed to add {g.user['email']} to audience: {e}")
+                logger.warning("Failed to add %s to nurture audience: %s", g.user["email"], e)
 
     lang = g.get("lang", "en")
     t = get_translations(lang)