Write the deployment manual, and keep the build context clean

docs/deployment.md is the server-side procedures: the compose file a server
runs, building and publishing a release, first deployment, reverse proxy,
upgrades and rollback, backups and restore, and a troubleshooting table.

The compose file lives in the manual rather than in the repository, because
the one at the root builds from source and is what development wants. The
server's pulls a published image, pins a release tag, and publishes the
admin port on the host's loopback instead of every interface — the panel is
Basic Auth and nothing else, so where that port is bound is the whole of its
security.

.dockerignore keeps the image build off storage/ (the live database and the
audio), .env (the admin password) and the leftover pgdata, which the build
cannot read anyway and which fails it outright.

README.md

@@ -18,6 +18,7 @@ Invite-only, no public registration. Built for about ten friends.
 | [docs/decisions.md](docs/decisions.md) | Why it is that way. Append-only |
 | [docs/theme.md](docs/theme.md) | The visual language: tokens, type, and what differs from the theme handoff |
 | [docs/later.md](docs/later.md) | Deliberately not in v1, with the reasoning kept |
+| [docs/deployment.md](docs/deployment.md) | Running it on a server: the compose file, releases, upgrades, backups |
 
 ## Branches and releases