Fix quarterly payment schedule to correct advance payment dates

- Q1: Due December 15 (previous year)
- Q2: Due March 15
- Q3: Due June 15
- Q4: Due September 15

Added new management command fix_quarterly_payment_schedule to update existing payments

app/static.backup.20250930_171758/README.md

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+# Static Files Directory
+
+This directory contains static files for the Django application.
+
+## Structure:
+- `css/` - Custom CSS files
+- `js/` - Custom JavaScript files  
+- `img/` - Image assets
+- `fonts/` - Font files
+
+Static files are collected to `staticfiles/` directory during deployment via `python manage.py collectstatic`.