feat(docker): rename web service to app, add nginx as web

Reorganize Docker Compose services: the Django/gunicorn container is now
`app` and nginx is `web`, better reflecting their roles. Add a dedicated
gunicorn configuration and install curl in the runtime image for health
checks.

Update documentation to reflect:
- Neo4j migration from ariel.incus to a dedicated umbriel.incus instance
- Rationale for requiring a dedicated Neo4j instance (single-tenancy
  assumptions, label/index isolation, schema ownership)
- New service naming in compose commands and log tailing examples

.env.example

@@ -0,0 +1,13 @@
+# =============================================================================
+# Mnemosyne — Docker Compose environment
+# =============================================================================
+# This file documents variables consumed by docker-compose.yaml itself
+# (image tags, port overrides, etc.). It is NOT the application config.
+#
+# Application config lives in mnemosyne/.env — copy mnemosyne/.env\ example
+# to mnemosyne/.env and fill in your values before running `docker compose up`.
+#
+# This file has no required variables for a default deployment: the compose
+# file uses a fixed image tag and port. Add overrides here if you parameterise
+# those in docker-compose.yaml (e.g. MNEMOSYNE_IMAGE, MNEMOSYNE_PORT).
+# =============================================================================