feat(budget): aylık bütçe motoru, limitler ve ay sonu tahmini

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/models/budget.ts

@@ -0,0 +1,32 @@
+import { model, models, Schema, type InferSchemaType, type Model } from "mongoose";
+
+/**
+ * Aylık bütçe (Bölüm 6.9): hane × bütçe ayı ("YYYY-MM") başına kategori limitleri.
+ * Ayın sınırları hane ayarı `budgetMonthStartDay`'e göre hesaplanır (belgede saklanmaz).
+ */
+const limitSchema = new Schema(
+  {
+    categoryId: { type: Schema.Types.ObjectId, required: true },
+    amountMinor: { type: Number, required: true, min: 0 },
+  },
+  { _id: false },
+);
+
+const budgetSchema = new Schema(
+  {
+    householdId: { type: Schema.Types.ObjectId, required: true },
+    month: { type: String, required: true, match: /^\d{4}-(0[1-9]|1[0-2])$/ },
+    limits: { type: [limitSchema], default: [] },
+    seedRef: { type: String, default: null },
+    createdBy: { type: Schema.Types.ObjectId, required: true },
+    updatedBy: { type: Schema.Types.ObjectId, required: true },
+  },
+  { timestamps: true, collection: "budgets" },
+);
+
+budgetSchema.index({ householdId: 1, month: 1 }, { unique: true });
+
+export type Budget = InferSchemaType<typeof budgetSchema>;
+
+export const BudgetModel: Model<Budget> =
+  (models.Budget as Model<Budget> | undefined) ?? model<Budget>("Budget", budgetSchema);