feat: add admin settings system and improve customer/pricing endpoints

Add centralized admin settings (company info, social links, quick calls,
sidebar visibility toggles, theme, logo upload) with singleton pattern
and full CRUD API. Add active/dedicated customer count endpoints for
dashboard stats. Fix automatic assignment route to use PUT instead of
GET. Refactor oil price endpoint to use schema serialization with null
safety.

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

app/stats/views.py

@@ -2,6 +2,7 @@ import logging
 from datetime import date
 from app.stats import stats
 import datetime
+from flask import request
 from app import db
 from app.common.responses import success_response
 from app.classes.delivery import Delivery_Delivery