Add dispute evidence storage API

[olalois]

Summary

Implement secure backend APIs for managing dispute evidence.

• Add multipart upload with file type, size, SHA-256 integrity, description, and metadata validation.
• Store encrypted files using server-generated opaque references.
• Restrict access to dispute participants and administrators.
• Add paginated evidence listing and soft-delete with encrypted file cleanup.
• Record upload, view, and delete actions in the activity audit log.
• Add the UUID-based dispute_evidence migration.

Closes #186

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