feat: 后端 CRUD 完成（episodes + yearly_targets + users 管理 API）

backend/app/models/yearly_target.py

@@ -0,0 +1,34 @@
+"""
+年度收视目标模型 — SQLModel
+
+业务规则（.clinerules 5.2 年度目标只增不改）:
+  - yearly_targets.year 有 UNIQUE 约束，数据库层已保证
+  - 代码端 INSERT 时只许 ON CONFLICT DO NOTHING，禁止 ON CONFLICT UPDATE
+  - 历史年份目标行绝不可被覆盖或删除
+  - 前端判颜色时按播出年份取对应年的目标，不是当前年份
+"""
+
+from datetime import date, datetime
+
+from sqlalchemy import Column
+from sqlalchemy import DateTime as SADateTime
+from sqlalchemy import Integer as SAInteger
+from sqlalchemy import Numeric as SANumeric
+from sqlalchemy.sql import func as sa_func
+from sqlmodel import Field, SQLModel
+
+
+class YearlyTarget(SQLModel, table=True):
+    __tablename__ = "yearly_targets"
+
+    id: int | None = Field(default=None, primary_key=True)
+    year: int = Field(unique=True, index=True)
+    base_target: float  # 基础目标
+    stretch_target: float  # 摸高目标
+    issued_date: date | None = Field(default=None)
+    notes: str | None = Field(default=None)
+
+    created_at: datetime | None = Field(
+        default=None,
+        sa_column=Column(SADateTime(timezone=True), nullable=False, server_default=sa_func.now()),
+    )