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.lock

@@ -97,6 +97,7 @@ GEM
     debug (1.11.1)
       irb (~> 1.10)
       reline (>= 0.3.8)
+    dotenv (3.2.0)
     drb (2.2.3)
     dry-cli (1.4.1)
     erb (6.0.7)
@@ -300,6 +301,7 @@ DEPENDENCIES
   brakeman
   bundler-audit
   debug
+  dotenv
   letter_opener_web
   pg (~> 1.1)
   puma (>= 5.0)