diff --git a/alembic/versions/0054_plugin_owner_id.py b/alembic/versions/0054_plugin_owner_id.py
new file mode 100644
index 0000000..4274dba
--- /dev/null
+++ b/alembic/versions/0054_plugin_owner_id.py
@@ -0,0 +1,49 @@
+"""Add owner_id to plugin entity tables for row-level ownership.
+
+Revision ID: 0054
+Revises: 0053
+Create Date: 2026-07-29
+
+This migration adds owner_id to plugin entity tables (files, folders,
+calendar_entries, calendars, tasks, subtasks) so that the universal
+visibility/permission system can be used for plugin entities.
+"""
+
+from alembic import op
+import sqlalchemy as sa
+from sqlalchemy.dialects.postgresql import UUID as PGUUID
+
+revision = "0054"
+down_revision = "0053"
+branch_labels = None
+depends_on = None
+
+# Plugin entity tables that need owner_id
+TABLES = [
+    "files",
+    "folders",
+    "calendar_entries",
+    "calendars",
+    "tasks",
+    "subtasks",
+]
+
+
+def upgrade() -> None:
+    for table in TABLES:
+        op.add_column(
+            table,
+            sa.Column(
+                "owner_id",
+                PGUUID(as_uuid=True),
+                sa.ForeignKey("users.id", ondelete="SET NULL"),
+                nullable=True,
+            ),
+        )
+        op.create_index(f"ix_{table}_owner", table, ["owner_id"])
+
+
+def downgrade() -> None:
+    for table in TABLES:
+        op.drop_index(f"ix_{table}_owner", table_name=table)
+        op.drop_column(table, "owner_id")
diff --git a/app/plugins/builtins/calendar/models.py b/app/plugins/builtins/calendar/models.py
index cec29ff..caca564 100644
--- a/app/plugins/builtins/calendar/models.py
+++ b/app/plugins/builtins/calendar/models.py
@@ -21,7 +21,7 @@ from app.core.db import Base, TenantMixin
 from app.models.owned_mixin import OwnedMixin
 
 
-class Calendar(Base, TenantMixin):
+class Calendar(Base, TenantMixin, OwnedMixin):
     """Calendar entity — tenant-scoped, soft-deletable, owned by a user."""
 
     __tablename__ = "calendars"