chore: add pg gem for production environment

- Introduced the 'pg' gem for PostgreSQL support in the production
  environment.
- Updated Gemfile to include it in the production group.
- Corresponding update in Gemfile.lock to ensure version 1.5.9 is
  used.

This change prepares the application to use PostgreSQL as the
primary database in production, which is essential for running
production-grade applications effectively.

Gemfile

@@ -61,6 +61,10 @@ group :development do
   gem "web-console"
 end
 
+group :production do
+  gem 'pg', '~> 1.5'
+end
+
 group :test do
   # Use system testing [https://guides.rubyonrails.org/testing.html#system-testing]
   gem "capybara"

Gemfile.lock

@@ -241,6 +241,7 @@ GEM
     parser (3.3.7.0)
       ast (~> 2.4.1)
       racc
+    pg (1.5.9)
     propshaft (1.1.0)
       actionpack (>= 7.0.0)
       activesupport (>= 7.0.0)
@@ -434,6 +435,7 @@ DEPENDENCIES
   jbuilder
   jsbundling-rails
   kamal
+  pg (~> 1.5)
   propshaft
   puma (>= 5.0)
   rails (~> 8.0.1)