Fix type mismatch in PostController authorization checks

[Copilot]

Authorization logic was comparing a User object with a string, causing type mismatch.

Changes

• Fixed three authorization checks in PostController (edit(), update(), delete() methods)
• Changed from $post->getAuthor() !== $user->getUsername() to $post->getAuthorId() !== $user->getId()

// Before: comparing User object with string
if (!$user->isAdmin() && !$user->isEditor() && $post->getAuthor() !== $user->getUsername())

// After: comparing IDs
if (!$user->isAdmin() && !$user->isEditor() && $post->getAuthorId() !== $user->getId())

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