Fix 113 hardening issues across entire Go backend

Security:
- Replace all binding: tags with validate: + c.Validate() in admin handlers
- Add rate limiting to auth endpoints (login, register, password reset)
- Add security headers (HSTS, XSS protection, nosniff, frame options)
- Wire Google Pub/Sub token verification into webhook handler
- Replace ParseUnverified with proper OIDC/JWKS key verification
- Verify inner Apple JWS signatures in webhook handler
- Add io.LimitReader (1MB) to all webhook body reads
- Add ownership verification to file deletion
- Move hardcoded admin credentials to env vars
- Add uniqueIndex to User.Email
- Hide ConfirmationCode from JSON serialization
- Mask confirmation codes in admin responses
- Use http.DetectContentType for upload validation
- Fix path traversal in storage service
- Replace os.Getenv with Viper in stripe service
- Sanitize Redis URLs before logging
- Separate DEBUG_FIXED_CODES from DEBUG flag
- Reject weak SECRET_KEY in production
- Add host check on /_next/* proxy routes
- Use explicit localhost CORS origins in debug mode
- Replace err.Error() with generic messages in all admin error responses

Critical fixes:
- Rewrite FCM to HTTP v1 API with OAuth 2.0 service account auth
- Fix user_customuser -> auth_user table names in raw SQL
- Fix dashboard verified query to use UserProfile model
- Add escapeLikeWildcards() to prevent SQL wildcard injection

Bug fixes:
- Add bounds checks for days/expiring_soon query params (1-3650)
- Add receipt_data/transaction_id empty-check to RestoreSubscription
- Change Active bool -> *bool in device handler
- Check all unchecked GORM/FindByIDWithProfile errors
- Add validation for notification hour fields (0-23)
- Add max=10000 validation on task description updates

Transactions & data integrity:
- Wrap registration flow in transaction
- Wrap QuickComplete in transaction
- Move image creation inside completion transaction
- Wrap SetSpecialties in transaction
- Wrap GetOrCreateToken in transaction
- Wrap completion+image deletion in transaction

Performance:
- Batch completion summaries (2 queries vs 2N)
- Reuse single http.Client in IAP validation
- Cache dashboard counts (30s TTL)
- Batch COUNT queries in admin user list
- Add Limit(500) to document queries
- Add reminder_stage+due_date filters to reminder queries
- Parse AllowedTypes once at init
- In-memory user cache in auth middleware (30s TTL)
- Timezone change detection cache
- Optimize P95 with per-endpoint sorted buffers
- Replace crypto/md5 with hash/fnv for ETags

Code quality:
- Add sync.Once to all monitoring Stop()/Close() methods
- Replace 8 fmt.Printf with zerolog in auth service
- Log previously discarded errors
- Standardize delete response shapes
- Route hardcoded English through i18n
- Remove FileURL from DocumentResponse (keep MediaURL only)
- Thread user timezone through kanban board responses
- Initialize empty slices to prevent null JSON
- Extract shared field map for task Update/UpdateTx
- Delete unused SoftDeleteModel, min(), formatCron, legacy handlers

Worker & jobs:
- Wire Asynq email infrastructure into worker
- Register HandleReminderLogCleanup with daily 3AM cron
- Use per-user timezone in HandleSmartReminder
- Replace direct DB queries with repository calls
- Delete legacy reminder handlers (~200 lines)
- Delete unused task type constants

Dependencies:
- Replace archived jung-kurt/gofpdf with go-pdf/fpdf
- Replace unmaintained gomail.v2 with wneessen/go-mail
- Add TODO for Echo jwt v3 transitive dep removal

Test infrastructure:
- Fix MakeRequest/SeedLookupData error handling
- Replace os.Exit(0) with t.Skip() in scope/consistency tests
- Add 11 new FCM v1 tests

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/services/onboarding_email_service.go

@@ -39,6 +39,7 @@ func generateTrackingID() string {
 
 // HasSentEmail checks if a specific email type has already been sent to a user
 func (s *OnboardingEmailService) HasSentEmail(userID uint, emailType models.OnboardingEmailType) bool {
+	// TODO(hardening): Replace with OnboardingEmailRepository.HasSentEmail()
 	var count int64
 	if err := s.db.Model(&models.OnboardingEmail{}).
 		Where("user_id = ? AND email_type = ?", userID, emailType).
@@ -51,6 +52,7 @@ func (s *OnboardingEmailService) HasSentEmail(userID uint, emailType models.Onbo
 
 // RecordEmailSent records that an email was sent to a user
 func (s *OnboardingEmailService) RecordEmailSent(userID uint, emailType models.OnboardingEmailType, trackingID string) error {
+	// TODO(hardening): Replace with OnboardingEmailRepository.Create()
 	email := &models.OnboardingEmail{
 		UserID:     userID,
 		EmailType:  emailType,
@@ -66,6 +68,7 @@ func (s *OnboardingEmailService) RecordEmailSent(userID uint, emailType models.O
 
 // RecordEmailOpened records that an email was opened based on tracking ID
 func (s *OnboardingEmailService) RecordEmailOpened(trackingID string) error {
+	// TODO(hardening): Replace with OnboardingEmailRepository.MarkOpened()
 	now := time.Now().UTC()
 	result := s.db.Model(&models.OnboardingEmail{}).
 		Where("tracking_id = ? AND opened_at IS NULL", trackingID).
@@ -84,6 +87,7 @@ func (s *OnboardingEmailService) RecordEmailOpened(trackingID string) error {
 
 // GetEmailHistory gets all onboarding emails for a specific user
 func (s *OnboardingEmailService) GetEmailHistory(userID uint) ([]models.OnboardingEmail, error) {
+	// TODO(hardening): Replace with OnboardingEmailRepository.FindByUserID()
 	var emails []models.OnboardingEmail
 	if err := s.db.Where("user_id = ?", userID).Order("sent_at DESC").Find(&emails).Error; err != nil {
 		return nil, err
@@ -105,11 +109,13 @@ func (s *OnboardingEmailService) GetAllEmailHistory(page, pageSize int) ([]model
 
 	offset := (page - 1) * pageSize
 
+	// TODO(hardening): Replace with OnboardingEmailRepository.CountAll()
 	// Count total
 	if err := s.db.Model(&models.OnboardingEmail{}).Count(&total).Error; err != nil {
 		return nil, 0, err
 	}
 
+	// TODO(hardening): Replace with OnboardingEmailRepository.FindAllPaginated()
 	// Get paginated results with user info
 	if err := s.db.Preload("User").
 		Order("sent_at DESC").
@@ -126,6 +132,7 @@ func (s *OnboardingEmailService) GetAllEmailHistory(page, pageSize int) ([]model
 func (s *OnboardingEmailService) GetEmailStats() (*OnboardingEmailStats, error) {
 	stats := &OnboardingEmailStats{}
 
+	// TODO(hardening): Replace with OnboardingEmailRepository.GetStats()
 	// No residence email stats
 	var noResTotal, noResOpened int64
 	if err := s.db.Model(&models.OnboardingEmail{}).
@@ -159,6 +166,7 @@ func (s *OnboardingEmailService) GetEmailStats() (*OnboardingEmailStats, error)
 	return stats, nil
 }
 
+// TODO(hardening): Move to internal/dto/responses/onboarding_email.go
 // OnboardingEmailStats represents statistics about onboarding emails
 type OnboardingEmailStats struct {
 	NoResidenceTotal  int64 `json:"no_residence_total"`
@@ -173,6 +181,7 @@ func (s *OnboardingEmailService) UsersNeedingNoResidenceEmail() ([]models.User,
 
 	twoDaysAgo := time.Now().UTC().AddDate(0, 0, -2)
 
+	// TODO(hardening): Replace with OnboardingEmailRepository.FindUsersWithoutResidence()
 	// Find users who:
 	// 1. Are verified
 	// 2. Registered 2+ days ago
@@ -201,6 +210,7 @@ func (s *OnboardingEmailService) UsersNeedingNoTasksEmail() ([]models.User, erro
 
 	fiveDaysAgo := time.Now().UTC().AddDate(0, 0, -5)
 
+	// TODO(hardening): Replace with OnboardingEmailRepository.FindUsersWithoutTasks()
 	// Find users who:
 	// 1. Are verified
 	// 2. Have at least one residence
@@ -325,6 +335,7 @@ func (s *OnboardingEmailService) sendNoTasksEmail(user models.User) error {
 // SendOnboardingEmailToUser manually sends an onboarding email to a specific user
 // This is used by admin to force-send emails regardless of eligibility criteria
 func (s *OnboardingEmailService) SendOnboardingEmailToUser(userID uint, emailType models.OnboardingEmailType) error {
+	// TODO(hardening): Replace with UserRepository.FindByID() (inject UserRepository)
 	// Load the user
 	var user models.User
 	if err := s.db.First(&user, userID).Error; err != nil {
@@ -362,6 +373,7 @@ func (s *OnboardingEmailService) SendOnboardingEmailToUser(userID uint, emailTyp
 	// If already sent before, delete the old record first to allow re-recording
 	// This allows admins to "resend" emails while still tracking them
 	if alreadySent {
+		// TODO(hardening): Replace with OnboardingEmailRepository.DeleteByUserAndType()
 		if err := s.db.Where("user_id = ? AND email_type = ?", userID, emailType).Delete(&models.OnboardingEmail{}).Error; err != nil {
 			log.Error().Err(err).Uint("user_id", userID).Str("email_type", string(emailType)).Msg("Failed to delete old onboarding email record before resend")
 		}