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()

internal/openapi/openapi.json

@@ -0,0 +1,27 @@
+{
+  "openapi": "3.0.3",
+  "info": {
+    "title": "AI Customer Service API",
+    "version": "0.1.0"
+  },
+  "paths": {
+    "/actuator/health": {
+      "get": {
+        "responses": {
+          "200": {
+            "description": "service health"
+          }
+        }
+      }
+    },
+    "/api/v1/customer-service/webhook": {
+      "post": {
+        "responses": {
+          "200": {
+            "description": "message accepted"
+          }
+        }
+      }
+    }
+  }
+}