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/handlers/notification_handler.go

@@ -7,6 +7,8 @@ import (
 	"github.com/labstack/echo/v4"
 
 	"github.com/treytartt/honeydue-api/internal/apperrors"
+	"github.com/treytartt/honeydue-api/internal/dto/responses"
+	"github.com/treytartt/honeydue-api/internal/i18n"
 	"github.com/treytartt/honeydue-api/internal/middleware"
 	"github.com/treytartt/honeydue-api/internal/services"
 )
@@ -87,7 +89,7 @@ func (h *NotificationHandler) MarkAsRead(c echo.Context) error {
 		return err
 	}
 
-	return c.JSON(http.StatusOK, map[string]interface{}{"message": "message.notification_marked_read"})
+	return c.JSON(http.StatusOK, responses.MessageResponse{Message: i18n.LocalizedMessage(c, "message.notification_marked_read")})
 }
 
 // MarkAllAsRead handles POST /api/notifications/mark-all-read/
@@ -102,7 +104,7 @@ func (h *NotificationHandler) MarkAllAsRead(c echo.Context) error {
 		return err
 	}
 
-	return c.JSON(http.StatusOK, map[string]interface{}{"message": "message.all_notifications_marked_read"})
+	return c.JSON(http.StatusOK, responses.MessageResponse{Message: i18n.LocalizedMessage(c, "message.all_notifications_marked_read")})
 }
 
 // GetPreferences handles GET /api/notifications/preferences/
@@ -200,7 +202,10 @@ func (h *NotificationHandler) UnregisterDevice(c echo.Context) error {
 		return apperrors.BadRequest("error.registration_id_required")
 	}
 	if req.Platform == "" {
-		req.Platform = "ios" // Default to iOS
+		return apperrors.BadRequest("error.platform_required")
+	}
+	if req.Platform != "ios" && req.Platform != "android" {
+		return apperrors.BadRequest("error.invalid_platform")
 	}
 
 	err = h.notificationService.UnregisterDevice(req.RegistrationID, req.Platform, user.ID)
@@ -208,7 +213,7 @@ func (h *NotificationHandler) UnregisterDevice(c echo.Context) error {
 		return err
 	}
 
-	return c.JSON(http.StatusOK, map[string]interface{}{"message": "message.device_unregistered"})
+	return c.JSON(http.StatusOK, responses.MessageResponse{Message: i18n.LocalizedMessage(c, "message.device_removed")})
 }
 
 // DeleteDevice handles DELETE /api/notifications/devices/:id/
@@ -225,7 +230,10 @@ func (h *NotificationHandler) DeleteDevice(c echo.Context) error {
 
 	platform := c.QueryParam("platform")
 	if platform == "" {
-		platform = "ios" // Default to iOS
+		return apperrors.BadRequest("error.platform_required")
+	}
+	if platform != "ios" && platform != "android" {
+		return apperrors.BadRequest("error.invalid_platform")
 	}
 
 	err = h.notificationService.DeleteDevice(uint(deviceID), platform, user.ID)
@@ -233,5 +241,5 @@ func (h *NotificationHandler) DeleteDevice(c echo.Context) error {
 		return err
 	}
 
-	return c.JSON(http.StatusOK, map[string]interface{}{"message": "message.device_removed"})
+	return c.JSON(http.StatusOK, responses.MessageResponse{Message: i18n.LocalizedMessage(c, "message.device_removed")})
 }