Fix race condition in Card::ActivitySpike::DetectionJob

The check-then-act pattern in `register_activity_spike` has been
replaced with `find_or_create_by!` to eliminate the race condition
that could lead to creating multiple activity spikes for a card. To
support this change, the `card_id` index on `cards_activity_spikes`
has been made unique.

ref: https://app.fizzy.do/5986089/cards/3063

db/migrate/20251120203100_add_unique_index_to_card_activity_spikes_on_card_id.rb

@@ -0,0 +1,17 @@
+class AddUniqueIndexToCardActivitySpikesOnCardId < ActiveRecord::Migration[8.2]
+  def change
+    reversible do |dir|
+      dir.up do
+        execute <<-SQL
+          DELETE s1 FROM card_activity_spikes s1
+          INNER JOIN card_activity_spikes s2
+          WHERE s1.card_id = s2.card_id
+          AND s1.updated_at < s2.updated_at
+        SQL
+      end
+    end
+
+    remove_index :card_activity_spikes, :card_id
+    add_index :card_activity_spikes, :card_id, unique: true
+  end
+end