fix(agent)：第 19 条 token 回填守卫兼容 SQL NULL 老行（token_plain IS NULL OR = ''）

internal/web/controller/agent.go

@@ -83,11 +83,16 @@ func (a *AgentController) findAgent(token string) (*model.AgentNode, bool) {
 	// Item 19 backfill: the token has just passed the hash check, so for
 	// rows created before plaintext storage existed, adopt the presented
 	// token as the recoverable copy. The guarded update only fires while
-	// token_plain is empty — an authenticated request can never overwrite
-	// an already-stored token (only issuance writes a fresh one).
+	// token_plain is unset — an authenticated request can never overwrite
+	// an already-stored token (only issuance writes a fresh one). "Unset"
+	// must cover SQL NULL as well as '': AutoMigrate added the column to
+	// pre-existing rows as NULL, and NULL = '' is never true, so an
+	// equality-only guard silently matched zero legacy rows in
+	// production (the GORM-scanned value reads as "" either way, which is
+	// why the in-memory check above cannot distinguish them).
 	if agent.TokenPlain == "" {
 		if res := database.GetDB().Model(&model.AgentNode{}).
-			Where("id = ? AND token_plain = ''", agent.Id).
+			Where("id = ? AND (token_plain IS NULL OR token_plain = '')", agent.Id).
 			Update("token_plain", token); res.Error == nil && res.RowsAffected > 0 {
 			agent.TokenPlain = token
 		}