feat: idle cash alert and category hierarchy (subcategories)

Two of the four ideas picked from the Maybe/Sure feature audit:

- Idle cash: a 5th proactive alert flags a depository account (checking/
  savings/cash) sitting at or above a configurable balance with no
  transaction touching it for a configurable number of days.
- Category hierarchy: categories can now have one level of subcategories
  (Groceries -> Alcohol). Dashboard/report aggregations roll a
  subcategory's spending into its parent's slice.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/alembic/versions/3b7c941bbadf_add_idle_cash_thresholds.py

@@ -0,0 +1,36 @@
+"""add idle cash thresholds to app_settings
+
+Revision ID: 3b7c941bbadf
+Revises: a71d9f0c3e58
+Create Date: 2026-08-25 10:00:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '3b7c941bbadf'
+down_revision: Union[str, None] = 'a71d9f0c3e58'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        'app_settings',
+        sa.Column('idle_cash_threshold_amount', sa.Numeric(14, 2), nullable=False, server_default='1000'),
+    )
+    op.add_column(
+        'app_settings',
+        sa.Column('idle_cash_threshold_days', sa.Integer(), nullable=False, server_default='60'),
+    )
+    op.alter_column('app_settings', 'idle_cash_threshold_amount', server_default=None)
+    op.alter_column('app_settings', 'idle_cash_threshold_days', server_default=None)
+
+
+def downgrade() -> None:
+    op.drop_column('app_settings', 'idle_cash_threshold_days')
+    op.drop_column('app_settings', 'idle_cash_threshold_amount')

backend/alembic/versions/8e2f5a13c9d4_add_category_parent_id.py

@@ -0,0 +1,32 @@
+"""add category parent_id (subcategories)
+
+Revision ID: 8e2f5a13c9d4
+Revises: 3b7c941bbadf
+Create Date: 2026-08-25 10:30:00.000000
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '8e2f5a13c9d4'
+down_revision: Union[str, None] = '3b7c941bbadf'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.add_column('categories', sa.Column('parent_id', sa.Integer(), nullable=True))
+    op.create_foreign_key(
+        'fk_categories_parent_id_categories', 'categories', 'categories', ['parent_id'], ['id'], ondelete='SET NULL'
+    )
+    op.create_index('ix_categories_parent_id', 'categories', ['parent_id'])
+
+
+def downgrade() -> None:
+    op.drop_index('ix_categories_parent_id', table_name='categories')
+    op.drop_constraint('fk_categories_parent_id_categories', 'categories', type_='foreignkey')
+    op.drop_column('categories', 'parent_id')