Add unique index backing AidStation uniqueness validation

[moveson]

Problem

AidStation validates uniqueness of split_id scoped to event_id, but the aid_stations table has no unique index backing it — only single-column non-unique indexes on event_id and split_id. Application-level uniqueness validation alone is subject to race conditions (two concurrent saves can both pass validation), so duplicates are technically possible.

This surfaced in #2121, where rubocop's Rails/UniqueValidationWithoutIndex flagged the validation and was silenced with an inline disable:

validates :split_id, uniqueness: { scope: :event_id, message: "only one of any given split permitted within an event" } # rubocop:disable Rails/UniqueValidationWithoutIndex, Layout/LineLength

Fix

1. Audit production for existing duplicate (event_id, split_id) pairs (and clean up if any exist — the migration will fail otherwise):

   select event_id, split_id, count(*) from aid_stations group by 1, 2 having count(*) > 1;

2. Migration: add_index :aid_stations, [:event_id, :split_id], unique: true (consider dropping the now-redundant single-column event_id index, since the composite index covers event_id lookups)
3. Remove the Rails/UniqueValidationWithoutIndex portion of the inline disable in app/models/aid_station.rb

Optional hardening while in there: event_id and split_id are also nullable at the DB level despite required belongs_to associations — could add null: false constraints in the same migration after confirming no null rows exist.

Note: event_group.rb, split.rb, and organization.rb carry the same inline disable for their own uniqueness validations; those are case-insensitive (lower(name)) or message-customized cases that may warrant their own follow-up.