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/championship.py

@@ -1,7 +1,7 @@
 import uuid
 from datetime import datetime
 
-from sqlalchemy import DateTime, Float, Integer, String, Text, Uuid, func
+from sqlalchemy import DateTime, Float, ForeignKey, Integer, String, Text, Uuid, func
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.database import Base
@@ -11,6 +11,7 @@ class Championship(Base):
     __tablename__ = "championships"
 
     id: Mapped[uuid.UUID] = mapped_column(Uuid(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    org_id: Mapped[uuid.UUID | None] = mapped_column(Uuid(as_uuid=True), ForeignKey("organizations.id", ondelete="SET NULL"))
     title: Mapped[str] = mapped_column(String(255), nullable=False)
     description: Mapped[str | None] = mapped_column(Text)
     location: Mapped[str | None] = mapped_column(String(500))
@@ -37,5 +38,6 @@ class Championship(Base):
         DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
     )
 
+    organization: Mapped["Organization | None"] = relationship(back_populates="championships")  # type: ignore[name-defined]
     registrations: Mapped[list["Registration"]] = relationship(back_populates="championship", cascade="all, delete-orphan")  # type: ignore[name-defined]
     participant_list: Mapped["ParticipantList | None"] = relationship(back_populates="championship", uselist=False, cascade="all, delete-orphan")  # type: ignore[name-defined]