feat: add theme preset system with admin CRUD, public listing, and user theme settings

- Add ChromaticColor (17 Tailwind colors) and NeutralColor (5 grays) enums
- Add ThemePreset table with flat color columns and unique name constraint
- Add admin theme endpoints (CRUD + set default) at /api/v1/admin/theme
- Add public theme listing at /api/v1/site/themes
- Add user theme settings (PATCH /theme) with color snapshot on User model
- User.color_* columns store per-user overrides; fallback to default preset then builtin
- Initialize default theme preset in migration
- Remove legacy defaultTheme/themes settings

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

sqlmodels/theme_preset.py

@@ -0,0 +1,117 @@
+from datetime import datetime
+from uuid import UUID
+
+from sqlmodel import Field
+
+from .base import SQLModelBase
+from .color import ChromaticColor, NeutralColor, ThemeColorsBase
+from .mixin import UUIDTableBaseMixin
+
+
+class ThemePresetBase(SQLModelBase):
+    """主题预设基础字段"""
+
+    name: str = Field(max_length=100)
+    """预设名称"""
+
+    is_default: bool = False
+    """是否为默认预设"""
+
+    primary: ChromaticColor
+    """主色调"""
+
+    secondary: ChromaticColor
+    """辅助色"""
+
+    success: ChromaticColor
+    """成功色"""
+
+    info: ChromaticColor
+    """信息色"""
+
+    warning: ChromaticColor
+    """警告色"""
+
+    error: ChromaticColor
+    """错误色"""
+
+    neutral: NeutralColor
+    """中性色"""
+
+
+class ThemePreset(ThemePresetBase, UUIDTableBaseMixin):
+    """主题预设表"""
+
+    name: str = Field(max_length=100, unique=True)
+    """预设名称（唯一约束）"""
+
+
+# ==================== DTO ====================
+
+class ThemePresetCreateRequest(SQLModelBase):
+    """创建主题预设请求 DTO"""
+
+    name: str = Field(max_length=100)
+    """预设名称"""
+
+    colors: ThemeColorsBase
+    """颜色配置"""
+
+
+class ThemePresetUpdateRequest(SQLModelBase):
+    """更新主题预设请求 DTO"""
+
+    name: str | None = Field(default=None, max_length=100)
+    """预设名称（可选）"""
+
+    colors: ThemeColorsBase | None = None
+    """颜色配置（可选）"""
+
+
+class ThemePresetResponse(SQLModelBase):
+    """主题预设响应 DTO"""
+
+    id: UUID
+    """预设UUID"""
+
+    name: str
+    """预设名称"""
+
+    is_default: bool
+    """是否为默认预设"""
+
+    colors: ThemeColorsBase
+    """颜色配置"""
+
+    created_at: datetime
+    """创建时间"""
+
+    updated_at: datetime
+    """更新时间"""
+
+    @classmethod
+    def from_preset(cls, preset: ThemePreset) -> 'ThemePresetResponse':
+        """从数据库模型转换为响应 DTO（平铺列 → 嵌套 colors 对象）"""
+        return cls(
+            id=preset.id,
+            name=preset.name,
+            is_default=preset.is_default,
+            colors=ThemeColorsBase(
+                primary=preset.primary,
+                secondary=preset.secondary,
+                success=preset.success,
+                info=preset.info,
+                warning=preset.warning,
+                error=preset.error,
+                neutral=preset.neutral,
+            ),
+            created_at=preset.created_at,
+            updated_at=preset.updated_at,
+        )
+
+
+class ThemePresetListResponse(SQLModelBase):
+    """主题预设列表响应 DTO"""
+
+    themes: list[ThemePresetResponse]
+    """主题预设列表"""