Move .env.example where Compose actually looks, and make it true

It sat in backend/ while Compose reads .env from the repository root, so
the "cp .env.example .env" in the compose header pointed at nothing.

It also documented four settings that Compose never forwarded to the
container: ALLOWED_ORIGINS and the three version-retention knobs would
have been set in .env and silently done nothing. They are passed through
now, unset meaning the server's own defaults. Checked both directions:
every variable Compose reads is documented, and nothing documented is
ignored.

Dropped PORT from the example — the bundled nginx proxies to 4000 by
name, so changing it could only break that. Added the data-directory and
uid/gid settings, which were read but undocumented.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

readme.md

@@ -42,13 +42,13 @@ A cross-platform note-taking application with rich text editing, freehand drawin
 inknote/
 ├── docker-compose.yml        # Deploy from the Gitea registry
 ├── docker-compose.dev.yml    # Build from source for local work
+├── .env.example              # Copy to .env — Compose reads it from here
 ├── build.sh                  # Build & push images to Gitea
 ├── backend/
 │   ├── server.js             # Express API + WebSocket server
 │   ├── sync-client.js        # Client-side sync engine library
 │   ├── package.json          # Node dependencies
-│   ├── Dockerfile            # Server container
-│   └── .env.example          # Environment variable template
+│   └── Dockerfile            # Server container
 ├── frontend/
 │   ├── src/
 │   │   ├── App.jsx           # Root: where the user is, which dialog is open