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>

config/database.yml

@@ -15,14 +15,17 @@
 default: &default
   adapter: postgresql
   encoding: unicode
-  # For details on connection pooling, see Rails configuration guide
-  # https://guides.rubyonrails.org/configuring.html#database-pooling
   max_connections: <%= ENV.fetch("RAILS_MAX_THREADS") { 5 } %>
-
+  # Connection details come from the environment, usually a .env file in
+  # the project directory (see .env.example). Unset = local socket, own user.
+  host: <%= ENV["NUMOLUS_DATABASE_HOST"] %>
+  port: <%= ENV["NUMOLUS_DATABASE_PORT"] %>
+  username: <%= ENV["NUMOLUS_DATABASE_USER"] %>
+  password: <%= ENV["NUMOLUS_DATABASE_PASSWORD"] %>
 
 development:
   <<: *default
-  database: numolus_development
+  database: <%= ENV.fetch("NUMOLUS_DATABASE_NAME", "numolus") %>
 
   # The specified database role being used to connect to PostgreSQL.
   # To create additional roles in PostgreSQL see `$ createuser --help`.
@@ -59,9 +62,8 @@ test:
   database: numolus_test
 
 # Production: one database for everything -- app data, Solid Queue, Solid
-# Cache and Solid Cable. The password comes from NUMOLUS_DATABASE_PASSWORD.
+# Cache and Solid Cable. Credentials via NUMOLUS_DATABASE_* (see .env.example).
 production:
   <<: *default
-  database: numolus_production
-  username: numolus
-  password: <%= ENV["NUMOLUS_DATABASE_PASSWORD"] %>
+  database: <%= ENV.fetch("NUMOLUS_DATABASE_NAME", "numolus") %>
+  username: <%= ENV.fetch("NUMOLUS_DATABASE_USER", "numolus") %>