fix(db): keep token_ledger when resetting usage data

[pacocartones]

Summary

• resetUsageHistory() no longer deletes token_ledger. The table is the record behind token balances and transfer idempotency keys, not usage analytics, so a usage reset must not change balances or free idempotency keys.
• deletedTokenLedger stays in the result and in the purge API response (always 0), so the response shape does not change.

Related Issues

• Closes fix(backend): preserve token balances when resetting usage data #16004.

Validation

• Change type: DB
• New test tests/unit/usage-history-reset-token-ledger.test.ts: fails on the base (ledger empty after the reset), passes with the change; tests/unit/usage-history-reset.test.ts still passes
• npm run lint (whole repo) and prettier --check on the touched files pass on the branch
• Reconciled with the current release base before opening

Tests Added Or Updated

• tests/unit/usage-history-reset-token-ledger.test.ts (new): seeds a grant and a transfer older than a day and runs resetUsageHistory("1d") and ("all"); asserts the rows, getBalance() and the idempotency of a retried transfer.

Coverage Notes

• Touches src/lib/db/cleanup.ts only (removes one reset target, documents one field); covered by the new test.

Reviewer Notes

• The Reset usage data dialog does not mention the ledger, so no UI copy changes.
• feat(api): add /v1/me/sessions/{id}/messages endpoint with opt-in turn capture #14864 also edits cleanup.ts and the purge route (adds deletedAgentSessionMessages) in other hunks; I will rebase if it lands first.
• Not done: a way to reset the ledger on purpose. If you want one, it should be its own explicit action.

[diegosouzapw]

Thanks @pacocartones — resetting usage history no longer deletes token_ledger, so balances survive a 1d or all reset. The response still reports deletedTokenLedger as 0. This is in the upcoming release.