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/scripts/demo_plan.py

@@ -124,8 +124,7 @@ def _build_input() -> tuple[AllocationInput, dict]:
     alloc_input = AllocationInput(
         today=TODAY,
         horizon_end=horizon_end,
-        planning_mode="couple",
-        age4_savable_cap=resolve_age4_savable_cap("couple", parent_count=2),
+        age4_savable_cap=resolve_age4_savable_cap(parent_count=2),
         children=[child],
         parents=[parent_1, parent_2],
         pools=pools,