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/user/register.py

@@ -0,0 +1,22 @@
+from typing import Literal
+
+from pydantic import BaseModel, EmailStr, field_validator
+
+
+class UserRegister(BaseModel):
+    email: EmailStr
+    password: str
+    full_name: str
+    phone: str | None = None
+    # Role requested at registration: 'member' or 'organizer'
+    requested_role: Literal["member", "organizer"] = "member"
+    # Organizer-only fields
+    organization_name: str | None = None
+    instagram_handle: str | None = None
+
+    @field_validator("organization_name")
+    @classmethod
+    def org_name_required_for_organizer(cls, v, info):
+        if info.data.get("requested_role") == "organizer" and not v:
+            raise ValueError("Organization name is required for organizer registration")
+        return v