refactor/fix: now using pgx errors for postgres error checking instead of trying to look up the error code;

feat: implemented working custom validation;
fix: authservice begin/complete registration

backend/internal/models/auth.go

@@ -23,13 +23,13 @@ type Tokens struct {
 }
 
 type RegistrationBeginRequest struct {
-	Username string  `json:"username" binding:"required,min=3,max=20"`
+	Username string  `json:"username" binding:"required,min=3,max=20" validate:"username"`
 	Email		 *string `json:"email" binding:"email"`
 	Password string  `json:"password" binding:"required"` // TODO: password checking
 }
 
 type RegistrationCompleteRequest struct {
-	Username 					string  `json:"username" binding:"required,min=3,max=20"`
+	Username 					string  `json:"username" binding:"required,min=3,max=20" validate:"username"`
 	VerificationCode	string  `json:"verification_code" binding:"required"`
 	Name							string  `json:"name" binding:"required,max=75"`
 	Birthday					*string `json:"birthday"`
@@ -41,7 +41,7 @@ type RegistrationCompleteResponse struct {
 }
 
 type LoginRequest struct {
-	Username string `json:"username" binding:"required,min=3,max=20"`
+	Username string `json:"username" binding:"required,min=3,max=20" validate:"username"`
 	Password string `json:"password" binding:"required,max=100"`
 	TOTP 		 *string `json:"totp"`
 }