BE-045: POST /sessions/{id}/reserve — book a spot with payment

Track: 1 — MVP
Section: 1.12 Live Session Lifecycle
Status: [ ] Todo
Repo: backend-api
Branch: feature/BE-045-reserve-session

Done When

Creates reservation with transaction_id. Deducts from wallet. Reservation count < max_participants enforced. Double-reservation returns 409

Notes

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