fix(bookings): enforce no-overlap atomically via exclusion constraints

Check-then-insert left a race: two concurrent requests for the same
slot could both confirm. Migration 00014 adds three partial btree_gist
exclusion constraints mirroring resourcesCollide semantics (same
staff, same location, both-unassigned), excluding cancelled rows and
class bookings. SQLSTATE 23P01 maps to ErrBookingConflict at all three
write paths so a lost race returns a clean 409.

Validated on a local Postgres 17 container: constraint matrix at SQL
level, repo-level 23P01 detection, concurrent burst yielding exactly
one confirmed booking.

Authored By: TDvorak <[email protected]>

Generated with [Devin](https://devin.ai)

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

apps/backend/internal/bookings/service.go

@@ -341,6 +341,9 @@ func (s *Service) Create(ctx context.Context, request domain.CreateBookingReques
 		Notes:          notes,
 	})
 	if err != nil {
+		if db.IsExclusionViolation(err) {
+			return domain.CreateBookingResponse{}, ErrBookingConflict
+		}
 		return domain.CreateBookingResponse{}, err
 	}