feat: implement image upload for posts (single image + compression)

[ammaarrahmed]

Summary

Add support for creating posts with a single image attachment, including upload optimization and reliable rendering in feed and detail views.

Scope

• Allow users to attach one image from camera or gallery during post creation.
• Compress/optimize image assets before upload to reduce payload size.
• Persist image URL in the post data model.
• Render attached images in post cards and post detail screens.

Acceptance Criteria

• Users can create and publish a post with exactly one image.
• Upload failures return clear, actionable user-facing messages.
• Existing text-only posts remain fully compatible.
• Tests cover model/provider handling for optional image URLs.

Roadmap Link

Phase 2 -> Post Enhancements