init: 管理会计OS初始代码

包含前后端完整代码：
- 前端：Vue3+Vite+ElementPlus
- 后端：FastAPI+SQLAlchemy
- 模块：驾驶舱/KPI/战略地图/预警/预算/成本/预测/改善行动
- 当前版本：v1.0.0

backend/app/models/budget_plan.py

@@ -0,0 +1,21 @@
+"""预算计划模型 — 管理会计OS"""
+from sqlalchemy import Column, Integer, String, Float, DateTime, ForeignKey, Text, func
+from app.database import Base
+
+
+class BudgetPlan(Base):
+    """预算计划 — 按KPI按月分解的目标值"""
+    __tablename__ = "budget_plans"
+
+    id = Column(Integer, primary_key=True, index=True)
+    kpi_id = Column(Integer, ForeignKey("kpi_definitions.id"), nullable=False, comment="关联KPI")
+    period = Column(String(20), nullable=False, comment="预算期间 2026-05")
+    budget_value = Column(Float, nullable=False, comment="预算值")
+    budget_year = Column(Integer, nullable=False, comment="预算年份")
+    budget_month = Column(Integer, nullable=False, comment="预算月份 1-12")
+    version = Column(String(20), default="v1.0", comment="版本号 v1.0/v2.0")
+    status = Column(String(20), default="active", comment="active/archived")
+    remark = Column(String(500), nullable=True, comment="备注")
+    created_by = Column(String(100), nullable=True)
+    created_at = Column(DateTime, server_default=func.now())
+    updated_at = Column(DateTime, server_default=func.now(), onupdate=func.now())