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/notification_service.go

@@ -4,9 +4,11 @@ import (
 	"context"
 	"encoding/json"
 	"errors"
+	"fmt"
 	"strconv"
 	"time"
 
+	"github.com/rs/zerolog/log"
 	"gorm.io/gorm"
 
 	"github.com/treytartt/honeydue-api/internal/apperrors"
@@ -184,8 +186,33 @@ func (s *NotificationService) GetPreferences(userID uint) (*NotificationPreferen
 	return NewNotificationPreferencesResponse(prefs), nil
 }
 
+// validateHourField checks that an optional hour value is in the valid range 0-23.
+func validateHourField(val *int, fieldName string) error {
+	if val != nil && (*val < 0 || *val > 23) {
+		return apperrors.BadRequest("error.invalid_hour").
+			WithMessage(fmt.Sprintf("%s must be between 0 and 23", fieldName))
+	}
+	return nil
+}
+
 // UpdatePreferences updates notification preferences
 func (s *NotificationService) UpdatePreferences(userID uint, req *UpdatePreferencesRequest) (*NotificationPreferencesResponse, error) {
+	// B-12: Validate hour fields are in range 0-23
+	hourFields := []struct {
+		value *int
+		name  string
+	}{
+		{req.TaskDueSoonHour, "task_due_soon_hour"},
+		{req.TaskOverdueHour, "task_overdue_hour"},
+		{req.WarrantyExpiringHour, "warranty_expiring_hour"},
+		{req.DailyDigestHour, "daily_digest_hour"},
+	}
+	for _, hf := range hourFields {
+		if err := validateHourField(hf.value, hf.name); err != nil {
+			return nil, err
+		}
+	}
+
 	prefs, err := s.notificationRepo.GetOrCreatePreferences(userID)
 	if err != nil {
 		return nil, apperrors.Internal(err)
@@ -256,7 +283,10 @@ func (s *NotificationService) UpdateUserTimezone(userID uint, timezone string) {
 	// Only update if timezone changed (avoid unnecessary DB writes)
 	if prefs.Timezone == nil || *prefs.Timezone != timezone {
 		prefs.Timezone = &timezone
-		_ = s.notificationRepo.UpdatePreferences(prefs)
+		if err := s.notificationRepo.UpdatePreferences(prefs); err != nil {
+			log.Error().Err(err).Uint("user_id", userID).Str("timezone", timezone).
+				Msg("Failed to update user timezone in notification preferences")
+		}
 	}
 }
 
@@ -430,6 +460,7 @@ func (s *NotificationService) UnregisterDevice(registrationID, platform string,
 
 // === Response/Request Types ===
 
+// TODO(hardening): Move to internal/dto/responses/notification.go
 // NotificationResponse represents a notification in API response
 type NotificationResponse struct {
 	ID               uint                     `json:"id"`
@@ -473,6 +504,7 @@ func NewNotificationResponse(n *models.Notification) NotificationResponse {
 	return resp
 }
 
+// TODO(hardening): Move to internal/dto/responses/notification.go
 // NotificationPreferencesResponse represents notification preferences
 type NotificationPreferencesResponse struct {
 	TaskDueSoon      bool `json:"task_due_soon"`
@@ -511,6 +543,7 @@ func NewNotificationPreferencesResponse(p *models.NotificationPreference) *Notif
 	}
 }
 
+// TODO(hardening): Move to internal/dto/requests/notification.go
 // UpdatePreferencesRequest represents preferences update request
 type UpdatePreferencesRequest struct {
 	TaskDueSoon      *bool `json:"task_due_soon"`
@@ -532,6 +565,7 @@ type UpdatePreferencesRequest struct {
 	DailyDigestHour      *int `json:"daily_digest_hour"`
 }
 
+// TODO(hardening): Move to internal/dto/responses/notification.go
 // DeviceResponse represents a device in API response
 type DeviceResponse struct {
 	ID             uint   `json:"id"`
@@ -569,6 +603,7 @@ func NewGCMDeviceResponse(d *models.GCMDevice) *DeviceResponse {
 	}
 }
 
+// TODO(hardening): Move to internal/dto/requests/notification.go
 // RegisterDeviceRequest represents device registration request
 type RegisterDeviceRequest struct {
 	Name           string `json:"name"`