Replace ineffective follow_own_posts migration with a working one

[trichoplax]

This deletes the migration db/migrate/20260208223211_follow_own_posts.rb and adds a new migration db/migrate/20260304104100_follow_own_posts.rb that uses unscoped Post to make the post author a follower of new threads for every post.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.74%. Comparing base (38c6dc0) to head (8e2b8ae).
⚠️ Report is 2 commits behind head on develop.

Additional details and impacted files

Components	Coverage Δ
controllers	75.85% <ø> (+0.03%)	⬆️
helpers	85.08% <ø> (ø)
jobs	66.88% <ø> (ø)
models	93.01% <ø> (ø)
tasks	61.11% <ø> (ø)

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