Create Weekend model in Prisma

[peterlipt]

Title: Create Weekend model in Prisma

Description:
• Add a new model called Weekend in apps/backend/prisma/schema.prisma.
• Fields:
• id: Integer, primary key, autoincremented
• name: String, required
• location: String, required
• track: String, optional
• seriesId: Integer, required (relation to Series)

Relations:
• A weekend belongs to one series.
• A weekend has multiple sessions (Session[]).

Acceptance Criteria:
• Prisma schema compiles without errors.
• prisma generate works after editing.