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/crud/crud_organization.py

@@ -0,0 +1,11 @@
+import uuid
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from app.models.organization import Organization
+
+
+async def get_by_user_id(db: AsyncSession, user_id: uuid.UUID) -> Organization | None:
+    result = await db.execute(select(Organization).where(Organization.user_id == user_id))
+    return result.scalar_one_or_none()