Implement RLS and security functions

Apply Row Level Security to orphaned_images and system_alerts tables. Create RLS policies for admin/moderator access. Replace system_health view with get_system_health() function.

src/integrations/supabase/types.ts

@@ -5989,15 +5989,6 @@ export type Database = {
         }
         Relationships: []
       }
-      system_health: {
-        Row: {
-          alerts_last_24h: number | null
-          checked_at: string | null
-          critical_alerts_count: number | null
-          orphaned_images_count: number | null
-        }
-        Relationships: []
-      }
     }
     Functions: {
       anonymize_user_submissions: {
@@ -6221,6 +6212,15 @@ export type Database = {
           updated_at: string
         }[]
       }
+      get_system_health: {
+        Args: never
+        Returns: {
+          alerts_last_24h: number
+          checked_at: string
+          critical_alerts_count: number
+          orphaned_images_count: number
+        }[]
+      }
       get_user_management_permissions: {
         Args: { _user_id: string }
         Returns: Json