📄(collaboration) add license notice for yhub-server directory

Signed-off-by: Kevin Jahns <kevin.jahns@protonmail.com>

src/yhub-server/LICENSE

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+License notice for the src/yhub-server directory
+================================================
+
+The source code in this directory is, like the rest of this repository,
+released under the MIT License (see the LICENSE file at the repository root,
+Copyright (c) 2023 Direction Interministérielle du Numérique - Gouvernement
+Français).
+
+Dependency on AGPL-licensed code
+--------------------------------
+
+However, the code in this directory (in particular `server.js`) depends on
+and runs in the same process as the `@y/hub` package ("yhub"), which is
+licensed under the GNU Affero General Public License v3.0 (AGPL-3.0) — or,
+alternatively, under a proprietary license available from its author.
+
+Unless you have obtained such a proprietary license for yhub, the combined
+work formed by yhub together with the code in this directory is governed by
+the terms of the AGPL-3.0. This means in particular:
+
+- If you modify the code in this directory and run the resulting server for
+  users over a network, or distribute it, you must license your
+  modifications under the AGPL-3.0 or an AGPL-compatible license and make
+  the corresponding source available (AGPL-3.0, section 13).
+- The MIT license of the files in this directory is compatible with this
+  obligation: MIT-licensed code may be incorporated into an AGPL-licensed
+  combined work.
+
+Scope
+-----
+
+This notice applies only to this directory. The rest of the La Suite Docs
+software (the Django backend, the frontend, and all other components in this
+repository) does not link against yhub and communicates with it exclusively
+through network requests (REST/HTTP and WebSocket). It therefore does not
+form a combined work with yhub and remains governed solely by the MIT
+License at the repository root.

src/yhub-server/README.md

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+# yhub-server
+
+This directory contains the La Suite Docs-specific configuration for
+[yhub](https://www.npmjs.com/package/@y/hub) (`@y/hub`), the collaboration
+server that synchronizes Yjs documents between editors in real time.
+
+It is not a fork of yhub — it is a thin wrapper (`server.js`) that:
+
+- starts a yhub instance (websocket sync on port 3002, backed by Redis/Valkey
+  and PostgreSQL),
+- plugs in an auth plugin that resolves users and per-document access rights
+  by calling the Docs Django backend (`/api/v1.0/users/me/` and
+  `/api/v1.0/documents/{id}/`),
+- mirrors the environment conventions used elsewhere in this repository
+  (`*_FILE` secret indirection, `COLLABORATION_SERVER_ORIGIN` allowlist, …).
+
+The `Dockerfile` builds the container image used by the `yhub` service in
+`compose.yml`.
+
+## ⚠️ License warning (AGPL)
+
+This directory depends on `@y/hub`, which is licensed under the
+**GNU AGPL-3.0** (or a separate proprietary license from its author). Unlike
+the rest of this repository (MIT), the code in this directory is loaded into
+the same process as AGPL-licensed code. As a consequence:
+
+- **Any modification to the code in this directory (in particular
+  `server.js`) must be released under an AGPL-compatible license** if you run
+  or distribute the resulting server, including making it available to users
+  over a network (AGPL section 13).
+- See the [LICENSE](./LICENSE) file in this directory for details.
+
+**The rest of La Suite Docs is not affected.** The Django backend and the
+frontend never link against yhub; they communicate with it exclusively
+through network requests (REST/HTTP and WebSocket). They remain under the
+MIT license of the repository root.