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.

README.md

@@ -49,6 +49,7 @@
 
 - **🤖 Integrated AI Editor** — CodeMirror 6 editor with AI toolbar: improve, correct, translate, generate, custom rewrite, toolbox (list, table, frontmatter, canvas) — multi-provider DeepSeek/OpenRouter/Gemini
 - **🧩 MCP Server & AI Agent** — Built-in Model Context Protocol server (`/mcp`) and tool-calling assistant: read, search and edit your vaults from Claude Desktop, Cursor… with two-step confirmations, per-vault permissions, rate limiting and secret redaction ([guide](docs/MCP_GUIDE.md))
+- **👥 Real-time Collaboration** — Simultaneous editing of the same document (Yjs/CRDT): colored remote cursors, presence indicator, conflict-free merge, automatic reconnection and server-side persistence ([details](docs/features/collaboration.md))
 - **🗺️ Interactive Graph View** — Canvas force-directed with Barnes-Hut O(n log n), filters (tag, type), depth, focus mode, navigation history ←→↑, export PNG, preview on hover (Ctrl+click)
 - **🗂️ Multi-vault** : View multiple Obsidian vaults simultaneously
 - **🌳 Tree Navigation** : Browse your folders and files in the sidebar
@@ -668,6 +669,22 @@ Lifecycle: Tauri spawns the Python backend → health check → opens the webvie
 
 ---
 
+## 👥 Real-time Collaboration
+
+Multiple users can edit the same markdown document simultaneously (Google Docs style):
+
+- **Conflict-free merge** via Yjs (CRDT): two people can type in the same place, no change is lost.
+- **Colored remote cursors** and visible selections in CodeMirror, labelled with each user's name.
+- **Presence indicator** in the editor header (avatars + connection status).
+- **Automatic reconnection** (exponential backoff): state is merged on return.
+- **Server-side persistence**: the document is written to disk 2 s after the last change.
+- **Transport**: WebSocket `ws(s)://<host>/ws/collab/{vault}/{path}`, authenticated via the
+  `access_token` cookie (or `?token=`) and subject to per-vault access control.
+
+No configuration is required: open the same file in two browsers (or two windows) to see it live.
+
+---
+
 ## 🔌 API
 
 ObsiGate exposes a complete REST API :