Add four service helpers used by the upcoming `/balance` overview:
- getSnapshotTotalsByDate(range?) — SUM(value) GROUP BY snapshot_date
with an optional inclusive [from, to] range. LEFT JOIN preserves
empty snapshots as zero rows so the chart shows continuity.
- getSnapshotTotalsByCategoryAndDate(range?) — same aggregation broken
down by balance_categories.key, returned as one row per snapshot
date with a `byCategory` map. Powers the stacked-area variant.
- getAccountsLatestSnapshot() — one row per active account with the
value of its most-recent snapshot line (NULL when none exists).
Filters archived accounts via WHERE is_active = 1 AND archived_at
IS NULL, matches the listBalanceAccounts default.
- getAccountsPeriodAnchor(range) — earliest snapshot_date >= from
per account, with the value at that date — the anchor used to
compute the per-account Δ% column on the accounts table.
Tests cover empty DB, single/multi snapshot, archived exclusion via
SQL inspection, date-range params (from-only, both bounds, open).
Refs: #141
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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