feat(savings): Add savings module with categories, transactions, recurring plans

- Categories: regular, deposits, credits, recurring, multi-user, accounts
- Transactions: deposits and withdrawals with user tracking
- Recurring plans: monthly payment obligations per user
- Stats: overdues calculation with allocation algorithm
- Excludes is_account categories from total sums
- Documentation: docs/SAVINGS.md

internal/model/habit.go

@@ -18,6 +18,8 @@ type Habit struct {
 	TargetCount     int            `db:"target_count" json:"target_count"`
 	ReminderTime    sql.NullString `db:"reminder_time" json:"-"`
 	ReminderTimeStr *string        `db:"-" json:"reminder_time"`
+	StartDate       sql.NullTime   `db:"start_date" json:"-"`
+	StartDateStr    *string        `db:"-" json:"start_date"`
 	IsArchived      bool           `db:"is_archived" json:"is_archived"`
 	CreatedAt       time.Time      `db:"created_at" json:"created_at"`
 	UpdatedAt       time.Time      `db:"updated_at" json:"updated_at"`
@@ -42,6 +44,12 @@ func (h *Habit) ProcessForJSON() {
 			h.ReminderTimeStr = &formatted
 		}
 	}
+	
+	// Process start_date
+	if h.StartDate.Valid {
+		formatted := h.StartDate.Time.Format("2006-01-02")
+		h.StartDateStr = &formatted
+	}
 }
 
 type HabitLog struct {
@@ -63,6 +71,7 @@ type CreateHabitRequest struct {
 	TargetDays   []int   `json:"target_days,omitempty"`
 	TargetCount  int     `json:"target_count,omitempty"`
 	ReminderTime *string `json:"reminder_time,omitempty"`
+	StartDate    *string `json:"start_date,omitempty"`
 }
 
 type UpdateHabitRequest struct {
@@ -74,6 +83,7 @@ type UpdateHabitRequest struct {
 	TargetDays   []int   `json:"target_days,omitempty"`
 	TargetCount  *int    `json:"target_count,omitempty"`
 	ReminderTime *string `json:"reminder_time,omitempty"`
+	StartDate    *string `json:"start_date,omitempty"`
 	IsArchived   *bool   `json:"is_archived,omitempty"`
 }