feat(pricing): date-ranged price lists and immutable appointment invoices

Section 12 and the fifth closing rule: changing a price never changes an
already-booked appointment.

The pricing chain already existed and worked. Two things were missing. Tariff only
carries a year, so a rate change starting in Mehr could not be expressed — PriceList
now takes an explicit date range and Tariff remains the layer beneath it. And an
appointment stored a single number, so after a price change or a discount nobody
could say what those 2,400,000 rials were made of.

Price resolution walks four layers per service and takes the first hit: branch
override, then the covering price list, then the yearly tariff, then the service's own
price. The last one is the guarantee that a date no list covers still returns a price
rather than zero or an exception. breakdown.sources reports which layer answered, so a
surprising number can be traced instead of guessed at.

Two calculation decisions worth stating. Tax is computed on the patient's share, not
the gross — a patient does not pay tax on the portion the insurer covers. And a
discount larger than the amount floors the total at zero rather than going negative,
because a negative balance would mean the clinic owes the patient money, which nothing
downstream is built to mean.

A branch-specific list deliberately does not count as overlapping a general one; it
takes precedence instead. Treating them as a conflict would have made per-branch
exceptions impossible to express. Lists have no effect until activated, so drafting
next quarter's prices cannot disturb today's.

PriceSnapshot has no setters and a unique key on appointment_id: a snapshot that can
be edited is not a snapshot, and two invoices for one appointment would be two truths.
Corrections are a new row plus voiding the old one. Invoices are written during
confirm with the prices of that moment — computing later would let a rate change
between booking and invoicing produce a different number, which is exactly what rule
five forbids.

12 tests. The one that matters is
testBookedAppointmentKeepsItsOriginalInvoiceAfterAPriceChange: book, double the
service price, watch quote return the new number while the appointment's invoice
returns the old one. Without it rule five is only a claim.

1220 tests / 3551 assertions. phpstan back at its 14-error baseline. Frozen slot
contract green.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/api/README.md

@@ -88,6 +88,7 @@ Only **digits** are translated — no characters are stripped, so `IR` in a sheb
 | [appointment-plan.md](appointment-plan.md) | Appointment segments and plan preview | 3 |
 | [appointment-availability.md](appointment-availability.md) | Multi-resource availability search | 2 |
 | [appointment-booking.md](appointment-booking.md) | Holds, confirmation and multi-resource occupancy | 4 |
+| [pricing.md](pricing.md) | Date-ranged price lists and appointment invoices | 8 |
 | [appointment.md](appointment.md) | Appointments & slot booking | 6 |
 | [appointment-settings.md](appointment-settings.md) | Weekly schedule, date overrides, holidays | 14 |
 | [payment.md](payment.md) | Payments (Mellat / Sep) | 5 |