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/list_item.py

@@ -0,0 +1,22 @@
+from datetime import datetime
+from typing import Any
+
+from pydantic import model_validator
+
+from app.schemas.registration.out import RegistrationOut
+
+
+class RegistrationListItem(RegistrationOut):
+    championship_title: str | None = None
+    championship_event_date: datetime | None = None
+    championship_location: str | None = None
+
+    @model_validator(mode="before")
+    @classmethod
+    def extract_championship(cls, data: Any) -> Any:
+        if hasattr(data, "championship") and data.championship:
+            champ = data.championship
+            data.__dict__["championship_title"] = champ.title
+            data.__dict__["championship_event_date"] = champ.event_date
+            data.__dict__["championship_location"] = champ.location
+        return data