Rework balances around per-parent days left and deduce the partner from FK

Manual entry now takes one parent's sjukpenningnivå, lägstanivå and
dubbeldagar left (plus optional reserved days) and derives the rest in
build_snapshot, shared with the scraper. The scraper also reads the
family panel and exposes a partner deduction; with two parents it is
saved as a 'deduced' snapshot (migration 002). Parent count is the single
source of entitlement: one member gets 480 days and 96 savable at age 4,
two get 240 and 48 each. Workspaces are capped at two parents.

Also lands the in-flight squash of migrations 002–011 into the initial
schema and the removal of planning_mode.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

backend/tests/unit/test_services/test_allocation_swap_fairness.py

@@ -87,7 +87,6 @@ def _jullov_wrap_scenario(*, parent_a_starved: bool):
     alloc_input = _build_input(
         today=date(2027, 1, 1),
         horizon_end=date(2028, 12, 31),
-        planning_mode="couple",
         children=[child],
         parents=[parent_a, parent_b],
         pools={