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/db.go

@@ -779,3 +779,59 @@ func (db *Db) TransferAllowance(fromId int, toId int, amount float64) error {
 
 	return tx.Commit()
 }
+
+func (db *Db) ExportAllData() (*ExportData, error) {
+	var err error
+	data := &ExportData{
+		Users:      make([]ExportUser, 0),
+		Allowances: make([]ExportAllowance, 0),
+		History:    make([]ExportHistory, 0),
+		Tasks:      make([]ExportTask, 0),
+	}
+
+	for row := range db.db.Query("select id, name, balance, weight from users").Range(&err) {
+		u := ExportUser{}
+		if err = row.Scan(&u.ID, &u.Name, &u.Balance, &u.Weight); err != nil {
+			return nil, err
+		}
+		data.Users = append(data.Users, u)
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	for row := range db.db.Query("select id, user_id, name, target, balance, weight, colour from allowances").Range(&err) {
+		a := ExportAllowance{}
+		if err = row.Scan(&a.ID, &a.UserID, &a.Name, &a.Target, &a.Balance, &a.Weight, &a.Colour); err != nil {
+			return nil, err
+		}
+		data.Allowances = append(data.Allowances, a)
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	for row := range db.db.Query("select id, user_id, timestamp, amount, description from history").Range(&err) {
+		h := ExportHistory{}
+		if err = row.Scan(&h.ID, &h.UserID, &h.Timestamp, &h.Amount, &h.Description); err != nil {
+			return nil, err
+		}
+		data.History = append(data.History, h)
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	for row := range db.db.Query("select id, name, reward, assigned, schedule, completed, next_run from tasks").Range(&err) {
+		t := ExportTask{}
+		if err = row.Scan(&t.ID, &t.Name, &t.Reward, &t.Assigned, &t.Schedule, &t.Completed, &t.NextRun); err != nil {
+			return nil, err
+		}
+		data.Tasks = append(data.Tasks, t)
+	}
+	if err != nil {
+		return nil, err
+	}
+
+	return data, nil
+}