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/routers/championships.py

@@ -38,10 +38,11 @@ async def get_championship(
 @router.post("", response_model=ChampionshipOut, status_code=status.HTTP_201_CREATED)
 async def create_championship(
     data: ChampionshipCreate,
-    _user: User = Depends(get_organizer),
+    user: User = Depends(get_organizer),
     db: AsyncSession = Depends(get_db),
 ):
-    return await crud_championship.create(db, data)
+    org_id = user.organization.id if user.organization else None
+    return await crud_championship.create(db, data, org_id=org_id)
 
 
 @router.patch("/{champ_id}", response_model=ChampionshipOut)