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/dto.go

@@ -86,3 +86,45 @@ type TransferRequest struct {
 	To     int     `json:"to"`
 	Amount float64 `json:"amount"`
 }
+
+type ExportUser struct {
+	ID      int     `json:"id"`
+	Name    string  `json:"name"`
+	Balance int64   `json:"balance"`
+	Weight  float64 `json:"weight"`
+}
+
+type ExportAllowance struct {
+	ID      int      `json:"id"`
+	UserID  int      `json:"userId"`
+	Name    string   `json:"name"`
+	Target  int64    `json:"target"`
+	Balance int64    `json:"balance"`
+	Weight  float64  `json:"weight"`
+	Colour  *int     `json:"colour"`
+}
+
+type ExportHistory struct {
+	ID          int    `json:"id"`
+	UserID      int    `json:"userId"`
+	Timestamp   int64  `json:"timestamp"`
+	Amount      int64  `json:"amount"`
+	Description string `json:"description"`
+}
+
+type ExportTask struct {
+	ID        int     `json:"id"`
+	Name      string  `json:"name"`
+	Reward    int64   `json:"reward"`
+	Assigned  *int    `json:"assigned"`
+	Schedule  *string `json:"schedule"`
+	Completed *int64  `json:"completed"`
+	NextRun   *int64  `json:"nextRun"`
+}
+
+type ExportData struct {
+	Users      []ExportUser      `json:"users"`
+	Allowances []ExportAllowance `json:"allowances"`
+	History    []ExportHistory   `json:"history"`
+	Tasks      []ExportTask      `json:"tasks"`
+}