feat(security): encrypt game and player names with env-based key (fix…

[Arunodoy18]

Summary

Implements application-level encryption for game and player names.

Closes #2232.

Changes

• Adds Cloak.Ecto for field-level encryption.
• Introduces Copi.Vault using AES-GCM.
• Encryption key loaded from COPI_ENCRYPTION_KEY environment variable.
• Transparent encryption/decryption at schema level.
• Includes migration to encrypt legacy plaintext data.

Security Improvements

• Prevents plaintext storage of sensitive names.
• Ensures encryption-at-rest.
• Uses authenticated encryption (AES-GCM).
• No hardcoded secrets.

Notes

• Backward compatible during transition.
• No changes to existing business logic.