Restore unique index on board_publications.key (#2568)

* Restore unique index on board_publications.key

The account_id rollout (fe6df7085) replaced the unique index on
board_publications.key with a non-unique composite (account_id, key)
index. This was unintentional — other tables in the same migration
preserved uniqueness (account_join_codes, tags) but this one did not.

Without global uniqueness on key, a crafted data import can insert a
duplicate publication key via insert_all! (which bypasses model
validations). Since find_by_published_key queries globally without
account scoping, this enables public board URL hijacking after the
legitimate owner unpublishes.

Restore the original unique index on key and keep a plain account_id
index for account-scoped queries.

* Reorder index operations: add unique index before dropping composite

On MySQL, DDL is non-transactional. If the unique index creation
fails (e.g. duplicate keys exist), the previous ordering would leave
the composite index already dropped. Adding the unique index first
means a failure leaves the database unchanged.

* Add schema dumps for both SQLite and MySQL

Dump both schema files after running the migration against each
adapter to keep the PR complete for CI.

db/migrate/20260218120000_restore_unique_index_on_board_publication_key.rb

@@ -0,0 +1,7 @@
+class RestoreUniqueIndexOnBoardPublicationKey < ActiveRecord::Migration[8.2]
+  def change
+    add_index :board_publications, :key, unique: true
+    add_index :board_publications, :account_id
+    remove_index :board_publications, [:account_id, :key]
+  end
+end