feat(backend): Implement file upload service for dispute evidence

[KuchiMercy]

Description

Dispute resolution requires evidence submission but no file upload service exists. multer is installed. Build a complete file upload module.

Packages: multer v1.4.5, @types/multer (installed) | Frontend: F-12

Acceptance Criteria

• Create UploadModule with controller, service, entity
• POST /disputes/:id/evidence endpoint
• Support: PNG, JPG, WebP, PDF, TXT, DOCX
• Server-side MIME validation via magic bytes
• Max 10MB per file, max 10 files per dispute
• Image thumbnails (200x200) via sharp
• Local filesystem storage with S3 adapter interface
• UUID filenames to prevent path traversal
• Evidence entity with all metadata fields
• GET /disputes/:id/evidence (list)
• GET /disputes/:id/evidence/:evidenceId/download
• DELETE for admin-only evidence removal
• Virus scanning hook (ClamAV interface)
• Clean up orphaned files
• Rate limiting: 20 uploads/hour/user
• Unit and E2E tests

Points: 150

[Jaydams]

@Jaydams has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello maintainer, I would love to be assigned this issue to work on.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Jaydams to this issue.

[drips-wave]

Congratulations, @Jaydams! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @Jaydams: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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