refactor: moved hashing logic into application layer for security;

fix: error handling in auth service for database;
refactor: removed redundant taken email check;
chore: removed todos that were completed/not needed;
fix: leaking transactions in complete registration and login on error;
refactor: got rid of txless requests during transactions;

backend/internal/models/auth.go

@@ -33,7 +33,6 @@ type RegistrationCompleteRequest struct {
 	VerificationCode	string  `json:"verification_code" binding:"required"`
 	Name							string  `json:"name" binding:"required" validate:"name"`
 	Birthday					*string `json:"birthday"`
-	AvatarUrl					*string `json:"avatar_url" binding:"http_url,max=255"`
 }
 
 type RegistrationCompleteResponse struct {