refactor: replace hardcoded credentials with env vars

- Use getenv() with RuntimeException for missing required vars
- Add .env.example template
- Add .env to .gitignore
- Fix missing return statement in config array

.env.example

@@ -0,0 +1,4 @@
+# Database configuration
+DB_DSN=odbc:Driver=FreeTDS; Server=localhost; Port=1218; Database=mydb
+DB_USER=myuser
+DB_PASSWORD=changeme