Make Place.user_id NOT NULL — complete migration period

[Freika]

Summary

Closes out the migration window opened in #2494 (April 2026). Place.user_id was left nullable during the 1.7.0 release so the backfill job could run without blocking deploys. The backfill migration (May 2026, BackfillPlacesUserIdJob) has had ~3 months to process; this PR enforces the constraint.

Changes:

• app/models/place.rb — removes optional: true from belongs_to :user (the comment said "Optional until Stage 2 NOT NULL"; this is Stage 2)
• db/migrate/20260815100000_add_places_user_id_not_null_check.rb — adds a check constraint with validate: false (no table lock; follows the same pattern as AddDatabaseUsersConstraints)
• db/migrate/20260815100001_validate_places_user_id_not_null.rb — validates the constraint, calls change_column_null :places, :user_id, false, then removes the temporary check constraint
• app/services/visits/create.rb — drops the now-redundant .where(places: { user_id: user.id }) clause from find_existing_place; the column-level NOT NULL plus belongs_to ownership provides the same guarantee

⚠️ Pre-merge gate

Before merging, confirm that no rows have a null user_id in the environment you're deploying to:

rails runner 'count = Place.where(user_id: nil).count; puts "NULL user_id places: #{count}"; exit(1) if count > 0'

If the count is non-zero the backfill job is still in progress (or stalled). Do not merge until it's zero — ValidatePlacesUserIdNotNull will fail at deploy time otherwise, which rolls back the migration and leaves the schema inconsistent.

The check constraint migration (20260815100000) is safe to run at any time — it only adds a not-yet-validated constraint and takes no lock. The validate migration (20260815100001) is the one that enforces the constraint and will fail fast if any null rows remain.

Migration sequence on deploy

rails db:migrate   # runs 20260815100000 (instant, no lock)
                   # then 20260815100001 (validate + NOT NULL)

Both migrations run inside the normal deploy. No separate rake task needed.

Test plan

• Confirm Place.where(user_id: nil).count returns 0 in staging/production before merging
• Run rails db:migrate against a staging database and verify both migrations succeed
• Verify Place.new(name: 'X', lonlat: ...).save raises ActiveRecord::RecordInvalid (user required)
• Verify Visits::Create.new(user, params).call still creates/finds places correctly

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