Admin - Diagnostics

Explains, for one family/student and a partially specified lesson, whether a booking could happen and what stops it.

📄️Why a family's student cannot book with a teacher

Diagnoses a partially specified booking request: which checks pass or fail, and, for every field left unset, which values would work. `verdict` is `bookable` when the request fully pins down one booking and nothing failed, `blocked` when at least one check failed, `underspecified` when nothing failed but a dimension was left open (never reported as bookable).