test(backend): verify snapshot totals reconcile with derived tables

[brentthomas248]

Closes #1730

Summary:

• Add SnapshotScheduler integrity coverage that seeds an in-memory better-sqlite3 ledger and reconciles snapshot counts and totals against derived-table sums.
• Capture bids, settlements, disputes, and notifications in derived-state snapshots instead of only invoice rows.
• Cover empty state, PII redaction, point-in-time mutation behavior, mid-batch marking, and retention pruning.

Tests:

• npm --prefix backend test -- snapshot --runInBand
• disposable clean clone: git apply patch; npm --prefix backend ci; npm --prefix backend test -- snapshot --runInBand
• git diff --check

Notes:

• npm --prefix backend run lint is unavailable because backend/package.json has no lint script.
• Full backend Jest and tsc checks still have existing unrelated baseline failures outside this snapshot change.