refactor: extract Models and Data into MoneyMap.Core shared library

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

MoneyMap.Core/Models/Budget.cs

@@ -0,0 +1,62 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace MoneyMap.Models;
+
+public enum BudgetPeriod
+{
+    Weekly = 0,
+    Monthly = 1,
+    Yearly = 2
+}
+
+/// <summary>
+/// Represents a spending budget for a category or total spending.
+/// When Category is null, this is a "Total" budget that tracks all spending.
+/// </summary>
+public class Budget
+{
+    [Key]
+    public int Id { get; set; }
+
+    /// <summary>
+    /// The category this budget applies to.
+    /// Null means this is a total spending budget (all categories combined).
+    /// </summary>
+    [MaxLength(100)]
+    public string? Category { get; set; }
+
+    /// <summary>
+    /// The budget limit amount (positive value).
+    /// </summary>
+    [Column(TypeName = "decimal(18,2)")]
+    public decimal Amount { get; set; }
+
+    /// <summary>
+    /// The time period for this budget (Weekly, Monthly, Yearly).
+    /// </summary>
+    public BudgetPeriod Period { get; set; } = BudgetPeriod.Monthly;
+
+    /// <summary>
+    /// When the budget becomes effective. Used to calculate period boundaries.
+    /// </summary>
+    public DateTime StartDate { get; set; } = DateTime.Today;
+
+    /// <summary>
+    /// Whether this budget is currently active.
+    /// </summary>
+    public bool IsActive { get; set; } = true;
+
+    /// <summary>
+    /// Optional notes about this budget.
+    /// </summary>
+    [MaxLength(500)]
+    public string? Notes { get; set; }
+
+    // Computed properties
+    [NotMapped]
+    public bool IsTotalBudget => Category == null;
+
+    [NotMapped]
+    public string DisplayName => Category ?? "Total Spending";
+}