feat(postbox): enforce unified inbox separation

src/govoplan_postbox/backend/db/models.py

@@ -167,6 +167,11 @@ class PostboxTemplateRevision(Base, TimestampMixin):
         default=dict,
         nullable=False,
     )
+    grouping_policy: Mapped[dict[str, Any]] = mapped_column(
+        JSON,
+        default=dict,
+        nullable=False,
+    )
     routing_policy: Mapped[dict[str, Any]] = mapped_column(
         JSON,
         default=dict,
@@ -1012,6 +1017,7 @@ class PostboxGroupingSource(Base, TimestampMixin):
     position: Mapped[int] = mapped_column(Integer, default=0, nullable=False)
 
     grouping: Mapped[PostboxGrouping] = relationship(back_populates="sources")
+    postbox: Mapped[Postbox] = relationship()
 
 
 class PostboxAccessEvent(Base, TimestampMixin):