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/models/__init__.py

@@ -1,4 +1,5 @@
 from app.models.user import User, RefreshToken
+from app.models.organization import Organization
 from app.models.championship import Championship
 from app.models.registration import Registration
 from app.models.participant import ParticipantList
@@ -7,6 +8,7 @@ from app.models.notification import NotificationLog
 __all__ = [
     "User",
     "RefreshToken",
+    "Organization",
     "Championship",
     "Registration",
     "ParticipantList",