Add basic health check endpoint

nise-backend/src/main/kotlin/com/nisemoe/nise/controller/HealthController.kt

@@ -0,0 +1,22 @@
+package com.nisemoe.nise.controller
+
+import org.springframework.http.ResponseEntity
+import org.springframework.web.bind.annotation.GetMapping
+import org.springframework.web.bind.annotation.RestController
+
+data class HealthResponse(
+    val healthy: Boolean,
+)
+
+val healthResponse = HealthResponse(
+    healthy = true,
+)
+
+@RestController
+class HealthController {
+    @GetMapping("/health")
+    fun healthCheck(): ResponseEntity<HealthResponse> {
+        return ResponseEntity.ok(healthResponse)
+    }
+}
+