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/__init__.py

@@ -22,6 +22,7 @@ from .policy import admin_policy_router
 from .share import admin_share_router
 from .task import admin_task_router
 from .user import admin_user_router
+from .theme import admin_theme_router
 from .vas import admin_vas_router
 
 
@@ -46,6 +47,7 @@ admin_router.include_router(admin_file_router)
 admin_router.include_router(admin_policy_router)
 admin_router.include_router(admin_share_router)
 admin_router.include_router(admin_task_router)
+admin_router.include_router(admin_theme_router)
 admin_router.include_router(admin_vas_router)
 
 # 离线下载 /api/admin/aria2