feat: add guest mode, bug fixes, and self-hosted config

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/scripts/012_add_guest_provider.sql

@@ -0,0 +1,3 @@
+-- Add 'guest' as a valid provider for guest mode login
+ALTER TABLE users DROP CONSTRAINT IF EXISTS users_provider_check;
+ALTER TABLE users ADD CONSTRAINT users_provider_check CHECK (provider IN ('google', 'github', 'guest'));