feat: implement secure item drop system with server-side validation

[Darrin-Lin]

This pull request introduces a new secure system for handling pending item and gold drops for players, along with related API and validation improvements. The changes add a new model to track pending drops, update serializers and endpoints to support secure claims, and enhance validation and testing to prevent exploits.

Secure pending drops system:

• Added a new PlayerPendingDrop model to record pending item or gold drops for players, including ownership, item data, gold amount, and timestamps. This model is managed via a new database migration and includes a string representation for easy debugging. [1] [2]

API and serializer updates:

• Updated the PlayerProfileUpdateSerializer to accept a list of claimed_drops, enabling clients to securely claim pending drops.
• Added a new PlayerDropView API endpoint and registered it under the /drop/ path to handle drop-related operations.

Validation and testing improvements:

• Added tests to ensure the profile update endpoint rejects excessive gold, HP, level jumps, and hacked item spawns, improving security against common exploits.…nd anti-cheat checks