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/championship/create.py

@@ -0,0 +1,19 @@
+from datetime import datetime
+
+from pydantic import BaseModel
+
+
+class ChampionshipCreate(BaseModel):
+    title: str
+    description: str | None = None
+    location: str | None = None
+    event_date: datetime | None = None
+    registration_open_at: datetime | None = None
+    registration_close_at: datetime | None = None
+    form_url: str | None = None
+    entry_fee: float | None = None
+    video_max_duration: int | None = None
+    judges: list[dict] | None = None       # [{name, bio, instagram}]
+    categories: list[str] | None = None
+    status: str = "draft"
+    image_url: str | None = None