Implement magazine system and inventory management

- Add level-based magazine system (3 mags at L1, 2 at L3-5, 1 at L6-8)
- Replace ammo/chargers with current_ammo/magazines/bullets_per_magazine
- Add inventory system for storing and using shop items
- Add Magazine item to shop (15 XP, adds 1 magazine)
- Auto-migrate existing players from old ammo system
- Auto-update magazines when players level up
- Fix method name bugs (get_player_level -> calculate_player_level)

shop.json

@@ -8,18 +8,11 @@
       "amount": 1
     },
     "2": {
-      "name": "Accuracy Boost",
-      "price": 20,
-      "description": "+10% accuracy",
-      "type": "accuracy",
-      "amount": 10
-    },
-    "3": {
-      "name": "Lucky Charm",
-      "price": 30,
-      "description": "+5% duck spawn chance",
-      "type": "luck",
-      "amount": 5
+      "name": "Magazine",
+      "price": 15,
+      "description": "1 extra magazine",
+      "type": "magazine",
+      "amount": 1
     }
   }
 }