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/admin/share/__init__.py

@@ -53,7 +53,7 @@ async def router_admin_get_share_list(
 )
 async def router_admin_get_share(
     session: SessionDep,
-    share_id: int,
+    share_id: UUID,
 ) -> ResponseBase:
     """
     获取分享详情。
@@ -99,7 +99,7 @@ async def router_admin_get_share(
 )
 async def router_admin_delete_share(
     session: SessionDep,
-    share_id: int,
+    share_id: UUID,
 ) -> ResponseBase:
     """
     删除分享。