feat(history): 消息搜索/编辑/删除 + 会话置顶(D2/D3/D5)

- history/message_ops.go(新): Search(会话内, q/after互斥, LIKE参数化+%/_转义+大小写不敏感, 严格限定会话doc) / UpdateMessage(返回更新后Message) / DeleteMessage / messageByID / escapeLike
- history/session_pin.go(新): SetPinned; 迁移0009 chat_sessions加pinned INTEGER DEFAULT 0; store.go SessionMeta.Pinned + createSessionsTable加列
- session_preview.go: 查询加s.pinned + ORDER BY pinned DESC,(type=main) DESC,created_at ASC
- gateway history_write.go(新,保持history.go≤500): handlePatchSession(pinned+title) / handlePatchMessage / handleDeleteMessage; server.go加PATCH/DELETE /api/v1/messages/{id} + /sessions/{id}置顶
- history.go: handleListMessages读q分流Search; 契约PATCH title/置顶
- D2会话内搜索(不做跨会话,隔离会话不外泄红线); D3不做重发端点(客户端编辑后再发); D5用pinned bool
- 测试(message_ops/history_messages/migrate_pinned), build/vet/test绿+3平台交叉编译; Docs/api-client v0.30
- 未做: 编辑/删除WS实时广播(message_edited/deleted), 下批

internal/services/channelbridge/channelbridge_test.go

@@ -279,9 +279,17 @@ func (c *captureHistory) ListAllLog(context.Context, int64, int) ([]history.Mess
 	return nil, nil
 }
 func (c *captureHistory) DeleteSession(context.Context, string) error { return nil }
+func (c *captureHistory) Search(context.Context, string, string, int64, int) ([]history.Message, error) {
+	return nil, nil
+}
+func (c *captureHistory) UpdateMessage(_ context.Context, id, content string) (history.Message, error) {
+	return history.Message{ID: id, Content: content}, nil
+}
+func (c *captureHistory) DeleteMessage(context.Context, string) error { return nil }
 func (c *captureHistory) RenameSession(context.Context, string, string) error {
 	return nil
 }
+func (c *captureHistory) SetPinned(context.Context, string, bool) error { return nil }
 func (c *captureHistory) NewSubSession(context.Context, string) (string, error) {
 	return "", nil
 }