Complete CRUD operations for habits

Implement all missing endpoints for full habit management:
- GET /api/v1/habits - List all user habits
- GET /api/v1/habits/{id} - Get specific habit
- PUT /api/v1/habits/{id} - Update habit
- DELETE /api/v1/habits/{id} - Archive habit (soft delete)
- GET /api/v1/habits/{id}/entries - Get habit entry history
- DELETE /api/v1/habits/{id}/entries/{date} - Unmark habit (soft delete entry)

All endpoints include:
- TDD approach with comprehensive test coverage
- JWT authentication and ownership validation
- Proper error handling (404, 403, 400, 500)
- Clean architecture with separated commands/queries

internal/application/commands/update_habit.go

@@ -0,0 +1,64 @@
+package commands
+
+import (
+	"context"
+	"strings"
+
+	"apocapoc-api/internal/domain/repositories"
+	"apocapoc-api/internal/shared/errors"
+)
+
+type UpdateHabitCommand struct {
+	HabitID       string
+	UserID        string
+	Name          string
+	Description   string
+	CarryOver     bool
+	TargetValue   *float64
+	SpecificDays  []int
+	SpecificDates []int
+}
+
+type UpdateHabitHandler struct {
+	habitRepo repositories.HabitRepository
+}
+
+func NewUpdateHabitHandler(habitRepo repositories.HabitRepository) *UpdateHabitHandler {
+	return &UpdateHabitHandler{
+		habitRepo: habitRepo,
+	}
+}
+
+func (h *UpdateHabitHandler) Handle(ctx context.Context, cmd UpdateHabitCommand) error {
+	// Validate input
+	if strings.TrimSpace(cmd.Name) == "" {
+		return errors.ErrInvalidInput
+	}
+
+	// Find existing habit
+	habit, err := h.habitRepo.FindByID(ctx, cmd.HabitID)
+	if err != nil {
+		return err
+	}
+
+	// Check ownership
+	if habit.UserID != cmd.UserID {
+		return errors.ErrUnauthorized
+	}
+
+	// Cannot update archived habits
+	if !habit.IsActive() {
+		return errors.ErrInvalidInput
+	}
+
+	// Update fields
+	habit.Name = cmd.Name
+	habit.Description = cmd.Description
+	habit.CarryOver = cmd.CarryOver
+	habit.TargetValue = cmd.TargetValue
+	habit.SpecificDays = cmd.SpecificDays
+	habit.SpecificDates = cmd.SpecificDates
+
+	// Save changes
+	return h.habitRepo.Update(ctx, habit)
+}