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_db_unit_of_work.py

@@ -45,8 +45,8 @@ async def _workspace_count(conn: aiosqlite.Connection) -> int:
 
 async def _insert_workspace(conn: aiosqlite.Connection, workspace_id: str) -> None:
     await conn.execute(
-        "INSERT INTO workspace (id, name, planning_mode, created_at) VALUES (?, ?, ?, ?)",
-        (workspace_id, "Test", "solo", datetime.now().isoformat()),
+        "INSERT INTO workspace (id, name, created_at) VALUES (?, ?, ?)",
+        (workspace_id, "Test", datetime.now().isoformat()),
     )