POL-127: Add organizations table and championship ownership

- Create organizations table with Alembic migration (3-phase: create table, migrate data, drop old column)
- Add org_id FK on championships linking to organizations
- Refactor all schemas into one-class-per-file packages (auth, championship, organization, participant, registration, user)
- Update CRUD layer with selectinload for organization relationships
- Update frontend types and components to use nested organization object
- Remove phantom Championship fields (subtitle, venue, accent_color) from frontend

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

backend/app/schemas/registration/__init__.py

@@ -0,0 +1,13 @@
+from app.schemas.registration.create import RegistrationCreate
+from app.schemas.registration.update import RegistrationUpdate
+from app.schemas.registration.out import RegistrationOut
+from app.schemas.registration.list_item import RegistrationListItem
+from app.schemas.registration.with_user import RegistrationWithUser
+
+__all__ = [
+    "RegistrationCreate",
+    "RegistrationUpdate",
+    "RegistrationOut",
+    "RegistrationListItem",
+    "RegistrationWithUser",
+]