refactor: 白板从画笔画板改为文本记事本

原实现是 Canvas 画笔画板，与「文字白板/记事本」需求不符。重做为纯文本实时协作：
- model：strokes JSON -> content TEXT + version INT（乐观锁）+ edit_count；
  schema/dao/service 同步重构，append_stroke/replace_strokes -> update_content。
- WS 协议：stroke -> edit（发完整文本，debounce 400ms）；init 下发 content/version。
  update 帧广播给他人，cleared 广播给所有人。
- 前端：canvas -> textarea；收到远端 update 用最长公共前后缀算变更区间，
  仅替换该区间并保留光标（区间前不动/后平移/内移末尾）；清空/复制文本按钮。
- schema.sql 更新 whiteboard 表 DDL；DB 旧表 DROP 重建（开发环境）。
- 测试脚本与 README 同步更新。

app/controllers/whiteboard_controller.py

@@ -86,18 +86,18 @@ def delete_whiteboard(
 
 @router.websocket("/ws/wb/{board_id}")
 async def whiteboard_ws(websocket: WebSocket, board_id: str) -> None:
-    """白板实时协作端点。
+    """白板实时协作端点（文本记事本）。
 
     协议（JSON 文本帧）：
         client -> server:
             {"type":"hello","client_id":"..."}   首帧（可选，未带则服务端生成）
             {"type":"ping"}                      心跳，服务端回 pong 并刷新计时
-            {"type":"stroke","stroke":{...}}     新增笔画，持久化并广播给他人
+            {"type":"edit","content":"..."}      debounce 后发完整文本，持久化并广播给他人
             {"type":"clear"}                     清空，持久化并广播给所有人
         server -> client:
-            {"type":"init","strokes":[...],"stroke_count":n}
+            {"type":"init","content":"...","version":n,"edit_count":m}
             {"type":"pong"}
-            {"type":"stroke","stroke":{...},"client_id":"..."}
+            {"type":"update","content":"...","version":n,"client_id":"..."}  文本变更
             {"type":"cleared","client_id":"..."}
             {"type":"error","msg":"..."}
     """
@@ -115,7 +115,6 @@ async def whiteboard_ws(websocket: WebSocket, board_id: str) -> None:
     client_id = _extract_client_id(first) or uuid.uuid4().hex[:12]
 
     # 校验 board_id 并加载白板（不存在则新建）
-    from ..database import get_session_local
     try:
         board = _with_db(lambda db: WhiteboardService(WhiteboardDAO(db), hub=hub).get_or_create(board_id))
     except HTTPException as exc:
@@ -128,8 +127,9 @@ async def whiteboard_ws(websocket: WebSocket, board_id: str) -> None:
     await hub.register(conn)
     await _safe_send(websocket, {
         "type": "init",
-        "strokes": board.strokes,
-        "stroke_count": board.stroke_count,
+        "content": board.content,
+        "version": board.version,
+        "edit_count": board.edit_count,
     })
 
     # 主循环：收消息 -> 处理 -> 广播
@@ -146,17 +146,22 @@ async def whiteboard_ws(websocket: WebSocket, board_id: str) -> None:
                 continue
             # 任何有效业务帧都视为活性证据
             conn.touch()
-            if mtype == "stroke":
-                stroke = msg.get("stroke") or {}
+            if mtype == "edit":
+                content = msg.get("content")
+                if not isinstance(content, str):
+                    await _safe_send(websocket, {"type": "error", "msg": "content 必须是字符串"})
+                    continue
                 try:
-                    _with_db(lambda db: WhiteboardService(WhiteboardDAO(db), hub=hub).append_stroke(board_id, stroke))
+                    out = _with_db(
+                        lambda db: WhiteboardService(WhiteboardDAO(db), hub=hub).update_content(board_id, content)
+                    )
                 except HTTPException as exc:
                     await _safe_send(websocket, {"type": "error", "msg": exc.detail})
                     continue
-                # 广播给他人（发送者本地已画，不回推）
+                # 广播给他人（发送者本地已更新，不回推）
                 await hub.broadcast(
                     board_id,
-                    {"type": "stroke", "stroke": stroke, "client_id": client_id},
+                    {"type": "update", "content": out.content, "version": out.version, "client_id": client_id},
                     exclude=conn,
                 )
             elif mtype == "clear":