feat: add registration attempt rate limiting with Redis

feat: prevent email enumeration by caching registration state
fix: correct Redis key formatting for session termination cache
refactor: improve registration flow with Redis cooldown checks
chore: add Redis caching for registration in-progress state

backend/internal/services/auth.go

@@ -68,7 +68,7 @@ func NewAuthService(_log *zap.Logger, _dbctx database.DbContext, _redis *redis.C
 	// FIXME: review possible problems due to a large pipeline request
 	pipe := _redis.Pipeline()
 	for _, guid := range guids {
-		if err := pipe.Set(ctx, fmt.Sprint("session:%s:is_terminated", guid), true, 0); err != nil {
+		if err := pipe.Set(ctx, fmt.Sprintf("session::%s::is_terminated", guid), true, 0); err != nil {
 			panic("Failed to cache terminated session: " + err.Err().Error()) 
 		}
 	}
@@ -95,7 +95,25 @@ func (a *authServiceImpl) RegistrationBegin(request models.RegistrationBeginRequ
 
   defer helper.RollbackOnError(err)
 
-	// TODO: check occupation with redis
+	if isInProgress, err := a.redis.Get(
+		context.TODO(),
+		fmt.Sprintf("email::%s::registration_in_progress",
+		request.Email),
+	).Bool(); err != nil {
+		if err != redis.Nil {
+			a.log.Error(
+				"Failed to look up cached registration_in_progress state of email as part of registration procedure",
+				zap.String("email", request.Email),
+				zap.Error(err))
+			return false, errs.ErrServerError
+		}
+		isInProgress = false
+	} else if isInProgress {
+		a.log.Warn(
+			"Attempted to begin registration on email that is in progress of registration or on cooldown",	
+			zap.String("email", request.Email))
+		return false, errs.ErrTooManyRequests
+	}
 
 	if occupationStatus, err = db.TXQueries.CheckUserRegistrationAvailability(db.CTX, database.CheckUserRegistrationAvailabilityParams{
 		Email: request.Email,
@@ -118,7 +136,19 @@ func (a *authServiceImpl) RegistrationBegin(request models.RegistrationBeginRequ
 
 	} else if occupationStatus.EmailBusy {
 		// Falsely confirm in order to avoid disclosing registered email addresses
-		// TODO: save this email into redis
+		if err := a.redis.Set(
+			context.TODO(),
+			fmt.Sprintf("email::%s::registration_in_progress", request.Email), 
+			true,
+			time.Duration(10 * time.Minute), // XXX: magic number
+		).Err(); err != nil {
+			a.log.Error(
+				"Failed to falsely set cache registration_in_progress state for email as a measure to prevent email enumeration",	
+				zap.String("email", request.Email),
+				zap.Error(err))
+			return false, errs.ErrServerError
+		}
+
 		a.log.Warn(
 			"Attempted registration for a taken email",
 			zap.String("email", request.Email),
@@ -211,6 +241,19 @@ func (a *authServiceImpl) RegistrationBegin(request models.RegistrationBeginRequ
 			zap.String("code", generatedCode))
 	}
 
+	if err := a.redis.Set(
+		context.TODO(),
+		fmt.Sprintf("email::%s::registration_in_progress", request.Email), 
+		true,
+		time.Duration(10 * time.Minute), // XXX: magic number
+	).Err(); err != nil {
+		a.log.Error(
+			"Failed to cache registration_in_progress state for email",	
+			zap.String("email", request.Email),
+			zap.Error(err))
+		return false, errs.ErrServerError
+	}
+
 	if err = helper.Commit(); err != nil {
 		a.log.Error(
 			"Failed to commit transaction",