Start with subscriptions on for everyone

To prevent people missing a notification they care about when we ship
the subscriptions feature, we'll start out with everything toggled on
(which matches the previous behaviour). People can then opt out as they
like.

db/migrate/20250224152047_create_subscriptions.rb

@@ -8,5 +8,11 @@ class CreateSubscriptions < ActiveRecord::Migration[8.1]
 
       t.index [ :subscribable_type, :subscribable_id, :user_id ], unique: true
     end
+
+    # Subscribe everyone to their current buckets to start with
+    execute "
+      insert into subscriptions (subscribable_type, subscribable_id, user_id, created_at, updated_at)
+      select 'Bucket', bucket_id, user_id, current_timestamp, current_timestamp from accesses;
+    "
   end
 end