docs: add sync protocols and WebSocket API documentation

Phase 2 part 7: Add real-time sync protocol documentation - KOReader Protocol: progress sync, metadata fetch - Kobo Protocol: markup sync, library fetch - WebSocket: real-time events (progress updates, conflicts) - Include message format examples for all protocols
2026-02-02 08:52:13 -05:00
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+# WebSocket Protocol
+
+Real-time sync events broadcast to connected clients.
+
+## Connect to WebSocket
+
+**Endpoint**: `WS /ws/sync?token=<token>`
+
+### Connection Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|-----------|-------------|
+| token | string | Yes | JWT authentication token |
+
+### Example Connection
+
+```javascript
+const ws = new WebSocket('wss://bookhoard.com/ws/sync?token=eyJhbG...');
+```
+
+## Message Format
+
+All messages are JSON objects with a `type` field.
+
+### Client → Server Messages
+
+#### Ping (Heartbeat)
+
+```json
+{
+  "type": "ping"
+}
+```
+
+Keep connection alive. Server responds with `pong`.
+
+### Server → Client Messages
+
+#### Progress Update
+
+```json
+{
+  "type": "progress_update",
+  "timestamp": "2026-01-31T10:00:00Z",
+  "data": {
+    "book_id": "uuid",
+    "progress": {
+      "percentage": 0.45678,
+      "epubcfi": "epubcfi(/6/4/2:15)",
+      "chapter": 3
+    },
+    "annotations": {}
+  },
+  "source_device": {
+    "id": "device-uuid",
+    "name": "My Kobo",
+    "type": "kobo"
+  }
+}
+```
+
+Broadcast when any device updates reading progress.
+
+#### Conflict Detected
+
+```json
+{
+  "type": "conflict",
+  "timestamp": "2026-01-31T10:00:00Z",
+  "data": {
+    "book_id": "uuid",
+    "conflict_id": "uuid",
+    "conflict_type": "progress"
+  }
+}
+```
+
+Broadcast when a sync conflict is detected.
+
+#### Pong
+
+```json
+{
+  "type": "pong"
+}
+```
+
+Server response to client `ping`.
+
+## Connection Management
+
+- **Heartbeat**: Send `ping` every 30 seconds
+- **Reconnect**: Use exponential backoff if connection drops
+- **Authentication**: Token must be valid for connection
+- **Rate Limits**: 60 messages/minute per connection
+
+## Error Handling
+
+```json
+{
+  "type": "error",
+  "message": "Invalid token",
+  "code": "AUTH_FAILED"
+}
+```
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->