fix: P0-1 RateLimiter并发写安全 + P0-2工单操作错误码区分 + P1 rows.Close修复

P0-1 (limits.go): Allow()方法改为全程使用写锁保护counters map读写，避免RLock写入时的data race
P0-2 (ticket_workflow.go+ticket_handler.go): Assign/Resolve/Close操作先查询ticket存在性和状态，返回明确的CS_TICKET_4001/CS_TKT_4002/CS_TICKET_4092/CS_TICKET_4093错误码，handler根据错误前缀路由HTTP状态码
P1-1 (ticket_store.go): 移除GetStats中3处手动rows.Close()，只保留defer Close()

go.mod

@@ -0,0 +1,5 @@
+module github.com/bridge/ai-customer-service
+
+go 1.22
+
+require github.com/lib/pq v1.10.9