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/app/services/workspace_service.py

@@ -82,16 +82,10 @@ class WorkspaceService:
         self,
         workspace_id: UUID,
         name: str | None = None,
-        planning_mode: str | None = None,
     ) -> Workspace:
         ws = await self.get_workspace(workspace_id)
         if name is not None:
             ws.name = name
-        if planning_mode is not None:
-            valid_modes = {"solo", "couple"}
-            if planning_mode not in valid_modes:
-                raise ValidationError(f"planning_mode must be one of {valid_modes}")
-            ws.planning_mode = planning_mode
         return await self._ws.update(workspace_id, ws)
 
     async def list_members(self, workspace_id: UUID) -> list[PersonalData]:
@@ -99,9 +93,21 @@ class WorkspaceService:
         return await self._pd.list_by_workspace(workspace_id)
 
     async def add_placeholder_parent(self, workspace_id: UUID, display_name: str) -> PersonalData:
-        """Create a parent profile without a login (solo planning for a partner)."""
+        """Create a parent profile without a login (solo planning for a partner).
+
+        Capped at 2 parents per workspace (a child has exactly two legal
+        parents in this model) — mirrors the same cap enforced on the
+        invitation-redemption path (`redeem_invitation`).
+        """
         _validate_display_name(display_name)
         await self.get_workspace(workspace_id)  # verify workspace exists
+        existing_members = await self._pd.list_by_workspace(workspace_id)
+        if len(existing_members) >= 2:
+            raise ValidationError(
+                "This workspace already has its maximum number of parents",
+                message_key="workspace.parents.add.maxReached",
+                data={},
+            )
         pd = PersonalData(
             uuid=uuid4(),
             workspace_id=workspace_id,