Add Redis-based persistence for POS cart and customer selection

Implemented Redis caching with 2-hour TTL for POS session data:

Backend changes:
- Added Redis cache configuration in settings.py
- Created save_cart() endpoint to persist cart state
- Added cart and customer loading from Redis in pos_terminal()
- Validates cart items (products/kits) still exist in DB
- Added REDIS_HOST, REDIS_PORT, REDIS_DB to .env

Frontend changes:
- Added saveCartToRedis() with 500ms debounce
- Cart auto-saves on add/remove/quantity change
- Added cart initialization from Redis on page load
- Enhanced customer button with two-line display and reset button
- Red X button appears only for non-system customers

Features:
- Cart persists across page reloads (2 hour TTL)
- Customer selection persists (2 hour TTL)
- Independent cart per user+warehouse combination
- Automatic cleanup of deleted items
- Debounced saves to reduce server load

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

myproject/myproject/settings.py

@@ -363,6 +363,20 @@ TENANT_ADMIN_NAME = env('TENANT_ADMIN_NAME')
 DEFAULT_AUTO_FIELD = 'django.db.models.BigAutoField'
 
 
+# ============================================
+# CACHE CONFIGURATION (Redis)
+# ============================================
+
+CACHES = {
+    'default': {
+        'BACKEND': 'django.core.cache.backends.redis.RedisCache',
+        'LOCATION': f'redis://{env("REDIS_HOST", default="localhost")}:{env("REDIS_PORT", default="6379")}/{env("REDIS_DB", default="0")}',
+        'KEY_PREFIX': 'myproject',  # Префикс для всех ключей
+        'TIMEOUT': 300,  # Таймаут по умолчанию (5 минут)
+    }
+}
+
+
 # ============================================
 # CELERY CONFIGURATION
 # ============================================