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/cache_service.go

@@ -2,9 +2,10 @@ package services
 
 import (
 	"context"
-	"crypto/md5"
 	"encoding/json"
 	"fmt"
+	"hash/fnv"
+	"sync"
 	"time"
 
 	"github.com/redis/go-redis/v9"
@@ -18,38 +19,55 @@ type CacheService struct {
 	client *redis.Client
 }
 
-var cacheInstance *CacheService
+var (
+	cacheInstance *CacheService
+	cacheOnce    sync.Once
+)
 
-// NewCacheService creates a new cache service
+// NewCacheService creates a new cache service (thread-safe via sync.Once)
 func NewCacheService(cfg *config.RedisConfig) (*CacheService, error) {
-	opt, err := redis.ParseURL(cfg.URL)
-	if err != nil {
-		return nil, fmt.Errorf("failed to parse Redis URL: %w", err)
+	var initErr error
+
+	cacheOnce.Do(func() {
+		opt, err := redis.ParseURL(cfg.URL)
+		if err != nil {
+			initErr = fmt.Errorf("failed to parse Redis URL: %w", err)
+			return
+		}
+
+		if cfg.Password != "" {
+			opt.Password = cfg.Password
+		}
+		if cfg.DB != 0 {
+			opt.DB = cfg.DB
+		}
+
+		client := redis.NewClient(opt)
+
+		// Test connection
+		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel()
+
+		if err := client.Ping(ctx).Err(); err != nil {
+			initErr = fmt.Errorf("failed to connect to Redis: %w", err)
+			// Reset Once so a retry is possible after transient failures
+			cacheOnce = sync.Once{}
+			return
+		}
+
+		// S-14: Mask credentials in Redis URL before logging
+		log.Info().
+			Str("url", config.MaskURLCredentials(cfg.URL)).
+			Int("db", opt.DB).
+			Msg("Connected to Redis")
+
+		cacheInstance = &CacheService{client: client}
+	})
+
+	if initErr != nil {
+		return nil, initErr
 	}
 
-	if cfg.Password != "" {
-		opt.Password = cfg.Password
-	}
-	if cfg.DB != 0 {
-		opt.DB = cfg.DB
-	}
-
-	client := redis.NewClient(opt)
-
-	// Test connection
-	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
-	defer cancel()
-
-	if err := client.Ping(ctx).Err(); err != nil {
-		return nil, fmt.Errorf("failed to connect to Redis: %w", err)
-	}
-
-	log.Info().
-		Str("url", cfg.URL).
-		Int("db", opt.DB).
-		Msg("Connected to Redis")
-
-	cacheInstance = &CacheService{client: client}
 	return cacheInstance, nil
 }
 
@@ -311,9 +329,10 @@ func (c *CacheService) CacheSeededData(ctx context.Context, data interface{}) (s
 		return "", fmt.Errorf("failed to marshal seeded data: %w", err)
 	}
 
-	// Generate MD5 ETag from the JSON data
-	hash := md5.Sum(jsonData)
-	etag := fmt.Sprintf("\"%x\"", hash)
+	// Generate FNV-64a ETag from the JSON data (faster than MD5, non-cryptographic)
+	h := fnv.New64a()
+	h.Write(jsonData)
+	etag := fmt.Sprintf("\"%x\"", h.Sum64())
 
 	// Store both the data and the ETag
 	if err := c.client.Set(ctx, SeededDataKey, jsonData, SeededDataTTL).Err(); err != nil {