Clarify (or add) a usage-recalculation trigger for vfolders written directly on the filesystem

## Problem

Direct-filesystem writes — by **BAIFS and by the existing tus upload path alike** — do not synchronously update `vfolders.cur_size` / `num_files` / `last_used`. The current source shows no push mechanism a direct-FS writer could call to keep those DB counters current, so they lag reality. (Backend quota enforcement at the quota-scope directory — XFS/NetApp/CephFS — is unaffected; this is only about the DB-tracked counters.)

## Request

Clarification and, if none exists, a small addition:

1. **What is the intended refresh mechanism** for these counters — periodic scan, storage-proxy usage report, something else? Documenting the SLA lets satellite services set correct expectations.
2. **Is there (or can there be) an explicit "recalculate usage for vfolder X" trigger** — an idempotent, on-demand recompute a satellite could optionally call after a batch of writes?

If such a trigger exists or is added, BAIFS would ship an **optional reconciliation hook** to invoke it, keeping the DB counters closer to the filesystem truth without making it a hot-path dependency.

## Rationale

- The counter drift is a pre-existing property of the tus path, not new to BAIFS; a documented SLA + optional trigger benefits both.
- Keeps v1 semantics honest ("DB counters lag; the filesystem is authoritative") while offering a supported path to reconcile.

## Citations (`6d680960d`)

- Direct-FS write counter gap and the v1 SLA: [docs/01 §2 mitigations table](../../01-architecture.md), [docs/07 Q5](../../07-risks-open-questions.md).
- Advisory `max_files`/`max_size` columns not enforced on the file path (parity with tus): [docs/07 Q5](../../07-risks-open-questions.md).

---
_Filed from the BAIFS project: see lablup/BAIFS#6 and `docs/spikes/upstream-asks/05-usage-accounting-trigger.md` for full context and citations._

Read docs/01-architecture.md §2 and docs/07-risks-open-questions.md Q5, then inspect the existing vfolder usage-accounting and tus paths referenced by the issue. Confirm the current refresh mechanism and SLA, and determine whether an idempotent on-demand trigger exists; done means the behavior is documented or the trigger contract is specified and validated by relevant tests.

Written by the indexing model from the issue text.