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_authz.py

@@ -596,11 +596,10 @@ class TestForeignWorkspaceIdIs404:
                 "workspace_id": str(two_workspaces.ws_b.id),
                 "child_id": str(two_workspaces.child_a.id),
                 "total_days": 480,
-                "days_used": 0,
+                "sjukpenning_left": 390,
+                "lagsta_niva_left": 90,
+                "dubbeldagar_left": 60,
                 "reserved_days_left": 90,
-                "transferable_days_left": 150,
-                "sjukpenning_used": 0,
-                "lagsta_niva_used": 0,
             },
         )
         assert resp.status_code == 404