Add convert-png-jpeg CLI task to convert non-transparent PNGs to JPEG in-place

[naaaaaaaaaaaf]

Motivation

• Provide a maintenance tool to convert non-transparent PNG media to JPEG to reduce storage and bandwidth while preserving existing URLs and S3 keys.
• Allow safe, resumable, and bounded runs with --limit and --min-id options and a --dry-run preview mode.
• Use libvips for efficient image processing and avoid reprocessing transparent images.

Description

• Adds a new CLI command convert-png-jpeg under Mastodon::CLI::media with options --limit, --min-id, and --dry-run and documentation in long_desc.
• Queries MediaAttachment records with file_file_name ending in .png, optionally resumes from min_id and limits the number processed with limit.
• Uses Vips::Image to detect an alpha channel and skips transparent images, converts non-transparent images to JPEG (Q: 85) into a Tempfile, and assigns the tempfile to the attachment while updating file metadata before saving.
• Tracks and logs counts for processed, converted, skipped transparent, and skipped missing files, and rescues Vips::Error to log processing errors.

Testing

• Ran the test suite with bundle exec rspec, and the test suite passed.

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Codex Task