Improve authorization, error handling, and post data aggregation

[hussein-m-kandil]

Summary

This PR enhances API correctness, security, and data richness by improving authorization checks, standardizing error responses, and enriching post-related aggregation logic.

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Key Improvements

Authorization & Error Handling

• Return 403 for mutation attempts by non-owner or non-admin users.
• Return 404 when requesting:
  • Profiles with invalid IDs.
  • Posts with invalid IDs.

Post & Interaction Enhancements

• Add followed flag to post, comment, and vote authors.
• Include upvotes/downvotes summary with post data.
• Support fetching posts from profiles followed by the current user.
• Aggregate full user/author data with post comments and votes.