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/create.py

@@ -0,0 +1,10 @@
+import uuid
+
+from pydantic import BaseModel
+
+
+class RegistrationCreate(BaseModel):
+    championship_id: uuid.UUID
+    category: str | None = None
+    level: str | None = None
+    notes: str | None = None