Add unique index to poll voters table

Note that Rails 7.1 changes `find_or_create_by!` so it calls
`create_or_find_by!` when no record is found, meaning we'll rarely get
`RecordNotUnique` exceptions when using this method during a race
condition.

Adding this index means we need to remove the uniqueness validation.
According to the `create_or_find_by` documentation [1]:

> Columns with unique database constraints should not have uniqueness
> validations defined, otherwise create will fail due to validation
> errors and find_by will never be called.

We're adding a test that checks what happens when using
`create_or_find_by!`.

Note that we're creating voters combining `create_with` with
`find_or_create_by!`. Using `find_or_create_by!(...)` with all
attributes (including non-key ones like `origin`) fails when a voter
already exists with different values, e.g. an existing `origin: "web"`
and an incoming `"booth"`. In this situation the existing record is not
matched and the unique index raises an exception.

`create_with(...).find_or_create_by!(user: ..., poll: ...)` searches by
the unique key only and applies the extra attributes only on creation.
Existing voters are returned unchanged, which is the intended behavior.

For the `take_votes_from` method, we're handling a (highly unlikely, but
theoretically possible) scenario where a user votes at the same time as
taking voters from another user. For that, we're doing something similar
to what `create_or_find_by!` does: we're updating the `user_id` column
inside a new transaction (using a new transactions avoids a
`PG::InFailedSqlTransaction` exception when there are duplicate
records), and deleting the existing voter when we get a
`RecordNotUnique` exception.

On `Poll::WebVote` we're simply raising an exception when there's
already a user who's voted via booth, because the `Poll::WebVote#update`
method should never be called in this case.

We still need to use `with_lock` in `Poll::WebVote`, but not due to
duplicate voters (`find_or_create_by!` method will now handle the unique
record scenario, even in the case of simultaneous transactions), but
because we use a uniqueness validation in `Poll::Answer`; this
validation would cause an error in simultaneous transactions.

[1] https://api.rubyonrails.org/v7.1/classes/ActiveRecord/Relation.html#method-i-create_or_find_by

spec/lib/tasks/polls_spec.rb

@@ -4,49 +4,6 @@ describe "polls tasks" do
   let(:poll) { create(:poll) }
   let(:user) { create(:user, :level_two) }
 
-  describe "polls:remove_duplicate_voters" do
-    before { Rake::Task["polls:remove_duplicate_voters"].reenable }
-
-    it "removes duplicate voters" do
-      second_user = create(:user, :level_two)
-
-      voter = create(:poll_voter, poll: poll, user: user)
-      second_voter = create(:poll_voter, poll: poll, user: second_user)
-      other_user_voter = create(:poll_voter, poll: poll, user: create(:user, :level_two))
-      other_poll_voter = create(:poll_voter, poll: create(:poll), user: user)
-
-      2.times { insert(:poll_voter, poll_id: poll.id, user_id: user.id) }
-      insert(:poll_voter, poll_id: poll.id, user_id: second_user.id)
-
-      expect(Poll::Voter.count).to eq 7
-
-      Rake.application.invoke_task("polls:remove_duplicate_voters")
-
-      expect(Poll::Voter.count).to eq 4
-      expect(Poll::Voter.all).to match_array [voter, second_voter, other_user_voter, other_poll_voter]
-    end
-
-    it "removes duplicate voters on tenants" do
-      create(:tenant, schema: "voters")
-
-      Tenant.switch("voters") do
-        poll = create(:poll)
-        user = create(:user, :level_two)
-
-        create(:poll_voter, poll: poll, user: user)
-        insert(:poll_voter, poll_id: poll.id, user_id: user.id)
-
-        expect(Poll::Voter.count).to eq 2
-      end
-
-      Rake.application.invoke_task("polls:remove_duplicate_voters")
-
-      Tenant.switch("voters") do
-        expect(Poll::Voter.count).to eq 1
-      end
-    end
-  end
-
   describe "polls:remove_duplicate_answers" do
     before { Rake::Task["polls:remove_duplicate_answers"].reenable }