Add data migration export/import endpoints

Add GET /api/export to the Go backend that dumps all users, allowances,
history, and tasks (including completed) as a single JSON snapshot.

Add POST /api/import to the Spring backend that accepts the same JSON,
wipes existing data, inserts all records with original IDs preserved via
native SQL, and resets PostgreSQL sequences to avoid future collisions.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/main.go

@@ -51,6 +51,16 @@ const DefaultDomain = "localhost:8080"
 // The domain that the server is reachable at.
 var domain = DefaultDomain
 
+func exportData(c *gin.Context) {
+	data, err := db.ExportAllData()
+	if err != nil {
+		log.Printf("Error exporting data: %v", err)
+		c.JSON(http.StatusInternalServerError, gin.H{"error": ErrInternalServerError})
+		return
+	}
+	c.IndentedJSON(http.StatusOK, data)
+}
+
 func getUsers(c *gin.Context) {
 	users, err := db.GetUsers()
 	if err != nil {
@@ -713,6 +723,7 @@ func start(ctx context.Context, config *ServerConfig) {
 	router.DELETE("/api/task/:taskId", deleteTask)
 	router.POST("/api/task/:taskId/complete", completeTask)
 	router.POST("/api/transfer", transfer)
+	router.GET("/api/export", exportData)
 
 	srv := &http.Server{
 		Addr:    config.Addr,