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>

routers/api/v1/site/__init__.py

@@ -1,7 +1,10 @@
 from fastapi import APIRouter
 
 from middleware.dependencies import SessionDep
-from sqlmodels import ResponseBase, Setting, SettingsType, SiteConfigResponse
+from sqlmodels import (
+    ResponseBase, Setting, SettingsType, SiteConfigResponse,
+    ThemePreset, ThemePresetResponse, ThemePresetListResponse,
+)
 from sqlmodels.setting import CaptchaType
 from utils import http_exceptions
 
@@ -41,6 +44,22 @@ def router_site_captcha():
     """
     http_exceptions.raise_not_implemented()
 
+@site_router.get(
+    path='/themes',
+    summary='获取主题预设列表',
+)
+async def router_site_themes(session: SessionDep) -> ThemePresetListResponse:
+    """
+    获取所有主题预设列表
+
+    无需认证，前端初始化时调用。
+    """
+    presets: list[ThemePreset] = await ThemePreset.get(session, fetch_mode="all")
+    return ThemePresetListResponse(
+        themes=[ThemePresetResponse.from_preset(p) for p in presets]
+    )
+
+
 @site_router.get(
     path='/config',
     summary='站点全局配置',