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/infrastructure/http/auth_handlers.go

@@ -65,7 +65,6 @@ func NewAuthHandlers(
 type RegisterRequest struct {
 	Email    string `json:"email"`
 	Password string `json:"password"`
-	Timezone string `json:"timezone"`
 }
 
 type LoginRequest struct {
@@ -100,7 +99,7 @@ type LogoutRequest struct {
 // @Produce json
 // @Param request body RegisterRequest true "Registration data (password requires: min 8 chars, uppercase, lowercase, digit, special char)"
 // @Success 201 {object} RegisterResponse "Returns user ID and message about next steps"
-// @Failure 400 {object} ValidationErrorResponse "Invalid input: email format, password requirements, or timezone"
+// @Failure 400 {object} ValidationErrorResponse "Invalid input: email format or password requirements"
 // @Failure 403 {object} ErrorResponse "Registration is closed"
 // @Failure 409 {object} ErrorResponse "Email already registered"
 // @Failure 500 {object} ErrorResponse "Internal server error"
@@ -115,7 +114,6 @@ func (h *AuthHandlers) Register(w http.ResponseWriter, r *http.Request) {
 	cmd := commands.RegisterUserCommand{
 		Email:    req.Email,
 		Password: req.Password,
-		Timezone: req.Timezone,
 	}
 
 	result, err := h.registerHandler.Handle(r.Context(), cmd)