Read database, host and SMTP settings from a .env file

- gem dotenv loads .env (and .env.<environment>[.local]) at startup; real
  environment variables take precedence.
- database.yml takes host, port, user, password and name from
  NUMOLUS_DATABASE_*; the database is called numolus by default.
- .env.example documents every setting; .env itself stays ignored.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Gemfile

@@ -37,6 +37,10 @@ group :development do
   gem "web-console"
 end
 
+# Settings and secrets (database, SMTP, host) from a .env file in the project
+# directory; real environment variables win -- see .env.example
+gem "dotenv"
+
 # Vite as the frontend build, integrated into Rails [https://vite-ruby.netlify.app]
 gem "vite_rails"