feat(themes): custom CSS themes per user (upload, paste, live)

- Theme manager in settings: upload a .css file or paste CSS, template
  download/copy/insert, edit and delete; custom themes show up in the
  gallery and the quick switchers
- Themes are injected live as <style id="mindsift-custom-theme">; the
  active one is cached so it applies on boot without flashing
- Backend: user_themes table and GET/POST /themes, DELETE /themes/{id}
  (per user, max 200 KB)
- Docs: theming, REST, database

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Doku/database.md

@@ -12,6 +12,7 @@ Die Datei `migrations/001_init.sql` wird vom Code **nicht** gelesen und ist vera
 | `users` | Konten | `email` (eindeutig), `username`, `password_hash`, `display_name`, `role` (`admin`/`member`), `is_active`, `token_version`, `alarm_channels` (kommagetrennt), `alarm_email`, `discord_webhook_url`, `custom_webhook_url`, `notify_before_minutes`, `avatar_url`, `organization_id` (leer = keine Organisation) |
 | `user_invites` | Einladungscodes | `code`, `email`, `role`, `organization_id`, `expires_at`, `used_by` |
 | `revoked_tokens` | Sperrliste ausgeloggter JWTs | `token_hash`, `expires_at` |
+| `user_themes` | Eigene CSS-Themes eines Benutzers | `user_id` (FK, `ON DELETE CASCADE`), `name`, `css_content` |
 | `api_keys` | Webhook-/API-Schlüssel | `key_prefix`, `key_hash` (SHA-256), `scopes` (nicht ausgewertet), `last_used_at` |
 | `system_settings` | Schlüssel/Wert für Instanz-Einstellungen | `sharing_mode`, `allow_registration`, `instance_name`, `smtp_*` |