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/monitoring/service.go

@@ -2,6 +2,7 @@ package monitoring
 
 import (
 	"io"
+	"sync"
 	"time"
 
 	"github.com/hibiken/asynq"
@@ -31,6 +32,8 @@ type Service struct {
 	logWriter      *RedisLogWriter
 	db             *gorm.DB
 	settingsStopCh chan struct{}
+	stopOnce       sync.Once
+	statsInterval  time.Duration
 }
 
 // Config holds configuration for the monitoring service
@@ -71,6 +74,7 @@ func NewService(cfg Config) *Service {
 		logWriter:      logWriter,
 		db:             cfg.DB,
 		settingsStopCh: make(chan struct{}),
+		statsInterval:  cfg.StatsInterval,
 	}
 
 	// Check initial setting from database
@@ -90,11 +94,11 @@ func (s *Service) SetAsynqInspector(inspector *asynq.Inspector) {
 func (s *Service) Start() {
 	log.Info().
 		Str("process", s.process).
-		Dur("interval", DefaultStatsInterval).
+		Dur("interval", s.statsInterval).
 		Bool("enabled", s.logWriter.IsEnabled()).
 		Msg("Starting monitoring service")
 
-	s.collector.StartPublishing(DefaultStatsInterval)
+	s.collector.StartPublishing(s.statsInterval)
 
 	// Start settings sync if database is available
 	if s.db != nil {
@@ -102,17 +106,19 @@ func (s *Service) Start() {
 	}
 }
 
-// Stop stops the monitoring service
+// Stop stops the monitoring service. It is safe to call multiple times.
 func (s *Service) Stop() {
-	// Stop settings sync
-	close(s.settingsStopCh)
+	s.stopOnce.Do(func() {
+		// Stop settings sync
+		close(s.settingsStopCh)
 
-	s.collector.Stop()
+		s.collector.Stop()
 
-	// Flush and close the log writer's background goroutine
-	s.logWriter.Close()
+		// Flush and close the log writer's background goroutine
+		s.logWriter.Close()
 
-	log.Info().Str("process", s.process).Msg("Monitoring service stopped")
+		log.Info().Str("process", s.process).Msg("Monitoring service stopped")
+	})
 }
 
 // syncSettingsFromDB checks the database for the enable_monitoring setting