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/auth/token_pair.py

@@ -0,0 +1,10 @@
+from pydantic import BaseModel
+
+from app.schemas.user import UserOut
+
+
+class TokenPair(BaseModel):
+    access_token: str
+    refresh_token: str
+    token_type: str = "bearer"
+    user: UserOut