Stable checkpoint — Hypothesis invariants 1/3/4/5 + PUT /products

Cumulative since stable-py-v0.6.4 :
- 9 Hypothesis property tests on Order/OrderLine invariants (ADR-0059) :
  * Invariant 1 : total = sum(qty * unit_price_at_order)
  * Invariant 3 : OrderLine.unit_price_at_order snapshot immutability
  * Invariant 4 : Order.can_transition_to (state machine)
  * Invariant 5 : OrderLine.can_transition_to (state machine)
- New module src/mirador_service/order/totals.py :: compute_total()
- PUT /products/{id} endpoint + 3 unit tests
- TASKS.md cleanup
- Ruff cleanups (RUF002 unicode + format)

Post-merge main pipeline #2480725432 green at 18:39.