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

db/migrate/20250814122241_add_unique_index_to_poll_voters.rb

@@ -0,0 +1,5 @@
+class AddUniqueIndexToPollVoters < ActiveRecord::Migration[7.1]
+  def change
+    add_index :poll_voters, [:user_id, :poll_id], unique: true
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.1].define(version: 2025_03_13_014205) do
+ActiveRecord::Schema[7.1].define(version: 2025_08_14_122241) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
   enable_extension "plpgsql"
@@ -1209,6 +1209,7 @@ ActiveRecord::Schema[7.1].define(version: 2025_03_13_014205) do
     t.index ["officer_assignment_id"], name: "index_poll_voters_on_officer_assignment_id"
     t.index ["poll_id", "document_number", "document_type"], name: "doc_by_poll"
     t.index ["poll_id"], name: "index_poll_voters_on_poll_id"
+    t.index ["user_id", "poll_id"], name: "index_poll_voters_on_user_id_and_poll_id", unique: true
     t.index ["user_id"], name: "index_poll_voters_on_user_id"
   end