Remove timezone from User model and pass as request parameter

Timezone is now sent from the client on each request that needs it,
instead of storing it in the database. This simplifies the model and
allows timezone to be dynamic (useful for traveling users).

Changes:
- Remove timezone field from User entity
- Remove timezone from user registration
- GET /habits/today now requires ?timezone= query param
- Add migration to drop timezone column from database
- Update related tests

internal/application/commands/register_user.go

@@ -17,7 +17,6 @@ import (
 type RegisterUserCommand struct {
 	Email    string
 	Password string
-	Timezone string
 }
 
 type RegisterUserResult struct {
@@ -57,7 +56,7 @@ func (h *RegisterUserHandler) Handle(ctx context.Context, cmd RegisterUserComman
 		return nil, errors.ErrRegistrationClosed
 	}
 
-	if err := validation.ValidateRegistration(cmd.Email, cmd.Password, cmd.Timezone); err != nil {
+	if err := validation.ValidateRegistration(cmd.Email, cmd.Password); err != nil {
 		return nil, fmt.Errorf("%w: %v", errors.ErrInvalidInput, err)
 	}
 
@@ -71,7 +70,7 @@ func (h *RegisterUserHandler) Handle(ctx context.Context, cmd RegisterUserComman
 		return nil, err
 	}
 
-	user := entities.NewUser(cmd.Email, hashedPassword, cmd.Timezone)
+	user := entities.NewUser(cmd.Email, hashedPassword)
 
 	emailVerificationRequired := false
 	if h.emailService != nil {