feat: collaboration temps reel - edition simultanee (#62)

WebSocket /ws/collab/{vault}/{path} (rooms par fichier), relais Yjs/CRDT, awareness (curseurs colores + presence), persistance serveur debounce 2s, auth WS + check_vault_access, reconnexion automatique. Frontend frontend/js/collab.js, backend backend/collab.py. Tests: 17 backend (5 clients simultanes) + 10 frontend. Docs: CHANGELOG, ROADMAP, fiche features/collaboration.md, README FR/EN.

backend/openapi_docs.py

@@ -33,6 +33,7 @@ TAGS_METADATA: list[dict[str, str]] = [
     {"name": "AI", "description": "AI-powered editor actions, provider status and model discovery."},
     {"name": "BooksLM", "description": "Directory-scoped AI chat (NotebookLM-style) over a vault folder."},
     {"name": "MCP", "description": "Model Context Protocol server (Streamable HTTP) exposing the shared AI tool layer to external clients (Claude Desktop, Cursor…)."},
+    {"name": "Collaboration", "description": "Real-time collaborative editing over WebSocket (`/ws/collab/{vault}/{path}`): Yjs/CRDT updates, awareness (cursors) and debounced server-side persistence."},
     {"name": "Sharing", "description": "Create and manage public read-only share links for documents."},
     {"name": "Webhooks", "description": "HTTP callbacks signed with HMAC-SHA256 for file events."},
     {"name": "Conflicts", "description": "Detect and resolve Syncthing sync-conflict files."},
@@ -57,6 +58,13 @@ Authorization: Bearer <access_token>
 The same token is also accepted as an HTTP-only cookie, so browser clients can
 simply use `credentials: "include"`.
 
+### Real-time collaboration
+Besides the REST API, a WebSocket endpoint `GET /ws/collab/{vault}/{path}` (upgrade) powers
+simultaneous editing of the same file: clients exchange Yjs/CRDT updates and awareness (remote
+cursors), and the server persists the document 2 s after the last change. Authentication uses the
+`access_token` cookie (or a `token` query parameter) and vault access is enforced per connection.
+See the collaboration feature documentation (`docs/features/collaboration.md`) for the protocol.
+
 ### Interactive documentation
 * **Swagger UI** — [/docs](/docs): try requests directly from the browser.
 * **ReDoc** — [/redoc](/redoc): clean, reading-oriented reference.