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/middleware/timezone.go

@@ -7,14 +7,25 @@ import (
 )
 
 const (
-	// TimezoneKey is the key used to store the user's timezone in the context
+	// TimezoneKey is the key used to store the user's timezone *time.Location in the context
 	TimezoneKey = "user_timezone"
+	// TimezoneNameKey stores the raw IANA timezone string from the request header
+	TimezoneNameKey = "user_timezone_name"
+	// TimezoneChangedKey is a bool context key indicating whether the timezone
+	// differs from the previously cached value for this user. Handlers should
+	// only persist the timezone to DB when this is true.
+	TimezoneChangedKey = "timezone_changed"
 	// UserNowKey is the key used to store the timezone-aware "now" time in the context
 	UserNowKey = "user_now"
 	// TimezoneHeader is the HTTP header name for the user's timezone
 	TimezoneHeader = "X-Timezone"
 )
 
+// package-level timezone cache shared across requests. It is safe for
+// concurrent use and has no TTL — entries are only updated when a new
+// timezone value is observed for a given user.
+var tzCache = NewTimezoneCache()
+
 // TimezoneMiddleware extracts the user's timezone from the request header
 // and stores a timezone-aware "now" time in the context.
 //
@@ -22,14 +33,31 @@ const (
 // or a UTC offset (e.g., "-08:00", "+05:30").
 //
 // If no timezone is provided or it's invalid, UTC is used as the default.
+//
+// The middleware also compares the incoming timezone with a cached value per
+// user and sets TimezoneChangedKey in the context so downstream handlers
+// know whether a DB write is needed.
 func TimezoneMiddleware() echo.MiddlewareFunc {
 	return func(next echo.HandlerFunc) echo.HandlerFunc {
 		return func(c echo.Context) error {
 			tzName := c.Request().Header.Get(TimezoneHeader)
 			loc := parseTimezone(tzName)
 
-			// Store the location and the current time in that timezone
+			// Store the location and the raw name in the context
 			c.Set(TimezoneKey, loc)
+			c.Set(TimezoneNameKey, tzName)
+
+			// Determine whether the timezone changed for this user so handlers
+			// can skip unnecessary DB writes.
+			changed := false
+			if tzName != "" {
+				if user := GetAuthUser(c); user != nil {
+					if !tzCache.GetAndCompare(user.ID, tzName) {
+						changed = true
+					}
+				}
+			}
+			c.Set(TimezoneChangedKey, changed)
 
 			// Calculate "now" in the user's timezone, then get start of day
 			// For date comparisons, we want to compare against the START of the user's current day
@@ -42,6 +70,20 @@ func TimezoneMiddleware() echo.MiddlewareFunc {
 	}
 }
 
+// IsTimezoneChanged returns true when the user's timezone header differs from
+// the previously observed value. Handlers should only persist the timezone to
+// DB when this returns true.
+func IsTimezoneChanged(c echo.Context) bool {
+	val, ok := c.Get(TimezoneChangedKey).(bool)
+	return ok && val
+}
+
+// GetTimezoneName returns the raw timezone string from the request header.
+func GetTimezoneName(c echo.Context) string {
+	val, _ := c.Get(TimezoneNameKey).(string)
+	return val
+}
+
 // parseTimezone parses a timezone string and returns a *time.Location.
 // Supports IANA timezone names (e.g., "America/Los_Angeles") and
 // UTC offsets (e.g., "-08:00", "+05:30").