first commit

Models/SubscriptionInstallment.cs

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+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Fitness_core_BE.Models
+{
+    /// <summary>
+    /// One scheduled part of a subscription's price (mostly-equal split, e.g. a
+    /// Rs. 6000 six-month plan → 6 monthly rows of Rs. 1000; the last row absorbs
+    /// rounding). Payments are allocated to installments in InstallmentNo order.
+    /// InstallmentCount = 1 plans get a single row due at onboarding.
+    /// </summary>
+    public class SubscriptionInstallment
+    {
+        [Key]
+        public Guid SubscriptionInstallmentId { get; set; }
+
+        public Guid FKSubscriptionId { get; set; }
+        [ForeignKey("FKSubscriptionId")]
+        public virtual CustomerSubscription Subscription { get; set; }
+
+        /// <summary>1-based position in the schedule.</summary>
+        public int InstallmentNo { get; set; }
+
+        public DateTime DueDate { get; set; }
+        public decimal Amount { get; set; }
+        public decimal PaidAmount { get; set; } = 0m;
+
+        /// <summary>"Pending" | "Partial" | "Paid".</summary>
+        public string Status { get; set; } = "Pending";
+
+        public bool IsActive { get; set; } = true;
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+    }
+}