Add version endpoint for backend

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

@@ -0,0 +1,19 @@
+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 VersionResponse(
+    val version: String,
+)
+
+val versionResponse = VersionResponse(
+    version = "v20250213",
+)
+
+@RestController
+class VersionController {
+    @GetMapping("/version")
+    fun getVersion(): ResponseEntity<VersionResponse> = ResponseEntity.ok(versionResponse)
+}