Handle duplicate legacy point backfills

[Freika]

GitHub issue: #3200

Summary

• removes exact legacy-only point duplicates before backfilling lonlat
• walks the large points table once in ID-ordered batches
• keeps affected user and import point counters accurate
• covers duplicate and normal backfill paths across batch boundaries

Verification

• focused regression: 1 example, 0 failures
• RuboCop: 2 files, no offenses
• full RSpec: 7,016 examples, 1 pre-existing failure in google_phone_takeout_invalid_utf8_spec.rb
• the same unrelated failure reproduces on OneDev dev at 1ec08149d20a83f10d0c1936f6028f48d36182e3

Review

• cycle 1 findings fixed: large-table cursor batching, import counter maintenance, release-note scope

Coordination

GitHub PR #3198 changes the same migration for an adjacent lock-timeout crash and will require merge-order coordination; it does not handle this duplicate-key failure.

[coderabbitai]

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✨ Finishing Touches

🧪 Generate unit tests (beta)

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• Commit unit tests in branch fix/legacy-lonlat-duplicates

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