first commit

Models/TenantCustomerLink.cs

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+using System.ComponentModel.DataAnnotations;
+
+namespace Fitness_core_BE.Models
+{
+    // Tenant-DB only, one row per customer this gym has ever enrolled. Every tenant-local
+    // table that used to FK straight to the central Customer.CusId keeps doing so, unchanged
+    // -- TenantCustomerLinkId IS the central Customer.CusId (same GUID value), not a new
+    // surrogate key. That's deliberate: a fresh surrogate key would mean "customerId" holds
+    // a different value depending on whether it's about to query a central or a tenant table,
+    // which is exactly the kind of silent, hard-to-spot bug this split needs to avoid. EF/MySQL
+    // still can't enforce this as a real cross-schema FK constraint -- it's just data -- but the
+    // ID space stays uniform everywhere, and this row is also the local anchor tenant tables
+    // join against for cached display fields without a central round-trip.
+    public class TenantCustomerLink
+    {
+        [Key]
+        public Guid TenantCustomerLinkId { get; set; } // == central Customer.CusId
+
+        // = the shared login User id (same GUID in Auth's Users table), cached.
+        public Guid CentralUserId { get; set; }
+
+        public string? DisplayName { get; set; }
+        public string? PrimaryPhoneCached { get; set; }
+        public string? EmailCached { get; set; }
+
+        // What a client-initiated delete flips. Central Customer.IsActive is never touched
+        // by a client-initiated delete -- see CustomerRepository.DeactivateCustomerForClient.
+        public bool IsActiveAtThisClient { get; set; } = true;
+
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+        public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+    }
+}