Configure Docker Compose through .env for Arcane

docker-compose.yml now reads every host-specific value (paths, user,
port, network, time zone) from .env with the previous values as
defaults. .env.example documents them; .env stays out of Git.

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

.gitignore

@@ -7,3 +7,6 @@ web/node_modules/
 web/dist/*
 !web/dist/.gitkeep
 web/*.tsbuildinfo
+
+# Local compose settings (see .env.example)
+.env