Add pluggable file storage abstraction for uploads

[nanaf6203-bit]

Problem

Avatar upload and property image upload use direct filesystem storage. No abstraction for S3, GCS, or Azure Blob Storage.

Expected Outcome

Pluggable storage provider for all file uploads, similar to signed URL providers.

Acceptance Criteria

• Storage provider interface
• Local filesystem provider (default)
• S3 provider implementation
• GCS provider implementation
• Drive by env var (STORAGE_PROVIDER)

Files Affected

src/properties/property-images.service.ts, src/users/avatar-upload.service.ts

Difficulty

Medium

Estimated Effort

Medium (1 week)

Labels

storage, enhancement, infrastructure