feat(pricing): scope a discount to members or to non-members, and let a branch-targeted one save at all

Membership tier decided only which base price was read; it was invisible to
every rule. A club wanting '10% off, members only' had no way to say it.
The nearest rule type, membership_duration, is tenure in months, which is a
different question and happily matches a non-member who has been around a
while. New membership_type rule type, with the CHECK constraint widened to
admit it — no existing row changes value, and the new one is unreachable
until a rule is authored with it.

The tier the rule matches is the same one that chose the base price, so a
discount and the price it discounts cannot disagree about who this is. An
unset tier reads as non_member, matching step 1, so a 'members only' rule
cannot quietly reach someone nobody ever classified.

And the bug the test for it found: NO branch-targeted discount could be
saved. pricing_rule_branches.academy_id is NOT NULL and the pivot has no
model, so no BelongsToAcademy hook filled it and a bare sync() died on a
not-null violation — every rule authored in the wizard with a branch
ticked. Stamped by hand, the way ProgramForm already does for
program_products.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>