BAT-008 Design

Schema and persistence design

Add manual_assets as a first-class table for non-transaction-derived assets/liabilities.
Add asset_snapshots as an append-only history table keyed by basis date plus evaluated item identity.
Migrations remain additive/idempotent; existing ledger/account flows must continue to operate when both tables are empty.

Account-backed assets use accounts.last_balance as the live authority source.
Non-transaction assets/liabilities use manual_assets as the live authority source.
asset_snapshots is a historical record layer, not a competing primary input source.
Net worth formula remains: asset buckets (cash, investment, deposit, real_estate, other) minus loan; missing valuations are excluded, not inferred.

Service layer should expose:
- manual assets list/report helpers
- basis-date asset snapshot read helpers
- net-worth timeseries helpers
- shared valuation/fallback helpers consumed by both MCP and dashboard
Snapshot read responses must expose basis date, bucket totals, liabilities, net worth, and inclusion/exclusion explanation fields.
Manual assets read responses must expose item name, value, basis date, liquidity_bucket, and memo/source metadata.

dashboard/app.py should consume the same valuation helpers as the MCP read tools.
If no manual assets/snapshots exist, dashboard behavior must safely fall back to REQ-005 account-based display.
Excluded/unvalued items should be explainable instead of silently counted.