feat(migrate): add AddIndexOnPostsUserId, down and AddSlugToPosts

examples/blog/db/migrate/20260908000003_create_claims.rb

@@ -0,0 +1,22 @@
+class CreateClaims < Prism::Orm::Migration
+  def up
+    create_table :claims do |t|
+      t.integer :post_id
+      t.integer :user_id
+      t.datetime :deleted_at
+      t.timestamps
+    end
+    # Belt and braces with Claim's own `validates :user_id, uniqueness:
+    # { scope: :post_id }`: the model-level check only ever sees rows this
+    # same process/transaction can already read, whereas this unique index
+    # is enforced by Store#validate_unique_indexes inside the very same
+    # transaction the insert itself runs in (see Store#insert), so it
+    # still catches a duplicate claim even in the hypothetical case of two
+    # inserts racing without going through Post#with_lock first.
+    add_index :claims, %i[post_id user_id], unique: true
+  end
+
+  def down
+    drop_table :claims
+  end
+end