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>

pyproject.toml

@@ -11,6 +11,7 @@ dependencies = [
     "argon2-cffi>=25.1.0",
     "asyncpg>=0.31.0",
     "cachetools>=6.2.4",
+    "captcha>=0.7.1",
     "fastapi[standard]>=0.122.0",
     "httpx>=0.27.0",
     "itsdangerous>=2.2.0",