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/color.py

@@ -1,7 +1,71 @@
+from enum import StrEnum
+
 from .base import SQLModelBase
 
-class ThemeResponse(SQLModelBase):
-    """主题响应 DTO"""
 
-    pass
-    
+class ChromaticColor(StrEnum):
+    """有彩色枚举（17种 Tailwind 调色板颜色）"""
+
+    RED = "red"
+    ORANGE = "orange"
+    AMBER = "amber"
+    YELLOW = "yellow"
+    LIME = "lime"
+    GREEN = "green"
+    EMERALD = "emerald"
+    TEAL = "teal"
+    CYAN = "cyan"
+    SKY = "sky"
+    BLUE = "blue"
+    INDIGO = "indigo"
+    VIOLET = "violet"
+    PURPLE = "purple"
+    FUCHSIA = "fuchsia"
+    PINK = "pink"
+    ROSE = "rose"
+
+
+class NeutralColor(StrEnum):
+    """无彩色枚举（5种灰色调）"""
+
+    SLATE = "slate"
+    GRAY = "gray"
+    ZINC = "zinc"
+    NEUTRAL = "neutral"
+    STONE = "stone"
+
+
+class ThemeColorsBase(SQLModelBase):
+    """嵌套颜色 DTO，API 请求/响应层使用"""
+
+    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"""
+
+
+BUILTIN_DEFAULT_COLORS = ThemeColorsBase(
+    primary=ChromaticColor.GREEN,
+    secondary=ChromaticColor.BLUE,
+    success=ChromaticColor.GREEN,
+    info=ChromaticColor.BLUE,
+    warning=ChromaticColor.YELLOW,
+    error=ChromaticColor.RED,
+    neutral=NeutralColor.ZINC,
+)