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/user.py

@@ -15,8 +15,6 @@ class User(Base):
     hashed_password: Mapped[str] = mapped_column(String(255), nullable=False)
     full_name: Mapped[str] = mapped_column(String(255), nullable=False)
     phone: Mapped[str | None] = mapped_column(String(50))
-    # Organizer-specific fields
-    organization_name: Mapped[str | None] = mapped_column(String(255))
     instagram_handle: Mapped[str | None] = mapped_column(String(100))
     role: Mapped[str] = mapped_column(String(20), nullable=False, default="member")
     # 'pending' | 'approved' | 'rejected'
@@ -30,6 +28,7 @@ class User(Base):
     refresh_tokens: Mapped[list["RefreshToken"]] = relationship(back_populates="user", cascade="all, delete-orphan")
     registrations: Mapped[list["Registration"]] = relationship(back_populates="user")  # type: ignore[name-defined]
     notification_logs: Mapped[list["NotificationLog"]] = relationship(back_populates="user")  # type: ignore[name-defined]
+    organization: Mapped["Organization | None"] = relationship(back_populates="user", uselist=False)  # type: ignore[name-defined]
 
 
 class RefreshToken(Base):