Add DAU statistics module

src/main/java/com/openisle/controller/StatController.java

@@ -0,0 +1,26 @@
+package com.openisle.controller;
+
+import com.openisle.service.UserVisitService;
+import lombok.RequiredArgsConstructor;
+import org.springframework.format.annotation.DateTimeFormat;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RequestParam;
+import org.springframework.web.bind.annotation.RestController;
+
+import java.time.LocalDate;
+import java.util.Map;
+
+@RestController
+@RequestMapping("/api/stats")
+@RequiredArgsConstructor
+public class StatController {
+    private final UserVisitService userVisitService;
+
+    @GetMapping("/dau")
+    public Map<String, Long> dau(@RequestParam(value = "date", required = false)
+                                 @DateTimeFormat(iso = DateTimeFormat.ISO.DATE) LocalDate date) {
+        long count = userVisitService.countDau(date);
+        return Map.of("dau", count);
+    }
+}