Persist reconciliation drift snapshots with trend query

[220nightmore-spec]

Closes #1713.

Summary

• add v013_reconciliation_snapshots with durable run_at, checked_count, drift_count, and severity fields plus a run_at index
• persist a snapshot at the end of each ReconciliationWorker run, including RPC-error reports
• add ReconciliationWorker.getDriftTrend(limit) with limit clamping and most-recent-first ordering
• expose the trend via GET /api/v1/admin/monitoring/reconciliation/trend?limit=...
• add focused Jest coverage for empty trends, persistence, ordering, limit clamping, and severity boundaries

Validation

• node .\node_modules\jest\bin\jest.js src\tests\reconciliation.test.ts --runInBand --forceExit
• direct v013 SQLite migration smoke: up creates the table/index, accepts a snapshot row, and down drops the table
• git diff --check

Notes

• I attempted the suggested migration dry-run, but the current migration runner fails before this migration with db.get is not a function because the runner adapter calls db.get() on a better-sqlite3 database instance. I left that shared runner behavior unchanged to keep this PR scoped to the reconciliation history feature.
• Full tsc --noEmit --pretty false is currently blocked by an existing syntax error in src/index.ts; the focused Jest coverage above passes.