diff --git a/backend/alembic/versions/a1d2a84b9abb_add_account_id_user_personalization.py b/backend/alembic/versions/a1d2a84b9abb_add_account_id_user_personalization.py
new file mode 100644
index 00000000..ca32f0d2
--- /dev/null
+++ b/backend/alembic/versions/a1d2a84b9abb_add_account_id_user_personalization.py
@@ -0,0 +1,45 @@
+"""add account_id to user personalization tables
+
+Revision ID: a1d2a84b9abb
+Revises: 7167e9374b0c
+Create Date: 2026-04-09 00:00:00.000000
+"""
+from typing import Sequence, Union
+from alembic import op
+import sqlalchemy as sa
+
+revision: str = 'a1d2a84b9abb'
+down_revision: Union[str, None] = '7167e9374b0c'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    for table in ('user_folders', 'user_pinned_trees'):
+        op.add_column(table, sa.Column('account_id', sa.UUID(), nullable=True))
+        op.create_foreign_key(
+            f'fk_{table}_account_id', table, 'accounts',
+            ['account_id'], ['id'], ondelete='CASCADE',
+        )
+        op.execute(f"""
+            UPDATE {table} t
+            SET account_id = u.account_id
+            FROM users u
+            WHERE t.user_id = u.id
+              AND t.account_id IS NULL
+        """)
+        result = op.get_bind().execute(
+            sa.text(f"SELECT COUNT(*) FROM {table} WHERE account_id IS NULL")
+        )
+        count = result.scalar()
+        if count > 0:
+            raise RuntimeError(f"ROLLBACK: {count} NULL account_id rows in {table}.")
+        op.alter_column(table, 'account_id', nullable=False)
+        op.create_index(f'ix_{table}_account_id', table, ['account_id'])
+
+
+def downgrade() -> None:
+    for table in ('user_folders', 'user_pinned_trees'):
+        op.drop_index(f'ix_{table}_account_id', table_name=table)
+        op.drop_constraint(f'fk_{table}_account_id', table, type_='foreignkey')
+        op.drop_column(table, 'account_id')
diff --git a/backend/app/models/folder.py b/backend/app/models/folder.py
index 7edaeaef..50923c86 100644
--- a/backend/app/models/folder.py
+++ b/backend/app/models/folder.py
@@ -46,6 +46,12 @@ class UserFolder(Base):
         nullable=False,
         index=True
     )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
     name: Mapped[str] = mapped_column(String(100), nullable=False)
     color: Mapped[str] = mapped_column(String(7), nullable=False, default="#6366f1")
     icon: Mapped[str] = mapped_column(String(50), nullable=False, default="folder")
diff --git a/backend/app/models/user_pinned_tree.py b/backend/app/models/user_pinned_tree.py
index c27edd08..d23b463a 100644
--- a/backend/app/models/user_pinned_tree.py
+++ b/backend/app/models/user_pinned_tree.py
@@ -24,6 +24,12 @@ class UserPinnedTree(Base):
         nullable=False,
         index=True
     )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
     tree_id: Mapped[uuid.UUID] = mapped_column(
         UUID(as_uuid=True),
         ForeignKey("trees.id", ondelete="CASCADE"),
diff --git a/backend/tests/test_phase1_migrations.py b/backend/tests/test_phase1_migrations.py
index 416ca4ab..a4f33de6 100644
--- a/backend/tests/test_phase1_migrations.py
+++ b/backend/tests/test_phase1_migrations.py
@@ -298,3 +298,48 @@ async def test_step_usage_log_account_id_is_logger_account(test_db: AsyncSession
     assert row.account_id == account.id, (
         f"account_id should be logger's account ({account.id}), got {row.account_id}"
     )
+
+
+# ── Group 4: User personalization ────────────────────────────────────────────
+
+@pytest.mark.asyncio
+async def test_user_folder_account_id_matches_user(test_db: AsyncSession):
+    """user_folders.account_id must match the owning user's account_id."""
+    from app.models.folder import UserFolder
+
+    account, user = await _make_account_and_user(test_db, "uf1")
+    folder = UserFolder(
+        user_id=user.id,
+        account_id=account.id,
+        name="My Folder",
+        color="#6366f1",
+        icon="folder",
+        display_order=0,
+    )
+    test_db.add(folder)
+    await test_db.commit()
+
+    result = await test_db.execute(select(UserFolder).where(UserFolder.id == folder.id))
+    row = result.scalar_one()
+    assert row.account_id == account.id
+
+
+@pytest.mark.asyncio
+async def test_user_pinned_tree_account_id_matches_user(test_db: AsyncSession):
+    """user_pinned_trees.account_id must match the pinning user's account_id."""
+    from app.models.user_pinned_tree import UserPinnedTree
+
+    account, user = await _make_account_and_user(test_db, "pt1")
+    tree = await _make_tree(test_db, account, user)
+    pin = UserPinnedTree(
+        user_id=user.id,
+        tree_id=tree.id,
+        account_id=account.id,
+        display_order=0,
+    )
+    test_db.add(pin)
+    await test_db.commit()
+
+    result = await test_db.execute(select(UserPinnedTree).where(UserPinnedTree.id == pin.id))
+    row = result.scalar_one()
+    assert row.account_id == account.id