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>

.env.example

@@ -0,0 +1,26 @@
+# numolus settings -- copy to .env (same directory) and fill in.
+# .env is read at startup (gem dotenv) and never committed. Variables that
+# are already set in the real environment take precedence over this file.
+# Environment-specific files are read too: .env.production, .env.development
+# (and *.local variants).
+
+# ---- Database (PostgreSQL) --------------------------------------------------
+# Unset host/user/password = local socket as the current OS user (development).
+NUMOLUS_DATABASE_NAME=numolus
+NUMOLUS_DATABASE_HOST=localhost
+NUMOLUS_DATABASE_PORT=5432
+NUMOLUS_DATABASE_USER=numolus
+NUMOLUS_DATABASE_PASSWORD=
+
+# ---- Server -------------------------------------------------------------------
+# Host name used in links in mails (sign-in, invitations)
+NUMOLUS_HOST=numolus.wicom.li
+# Rails secret (production): generate with  bin/rails secret
+SECRET_KEY_BASE=
+
+# ---- Mail (production; development shows mails at /letter_opener) ------------
+NUMOLUS_MAIL_FROM="numolus <noreply@wicom.li>"
+SMTP_ADDRESS=smtp.example.com
+SMTP_PORT=587
+SMTP_USER_NAME=
+SMTP_PASSWORD=