latency(P6): parallelize fetch_folder_grants per-folder reads #39

Priority: P6 (latency) — Tier 2. Part of the 2026-07-11 DynamoDB query-cost review.

Problem

routes/documents.rs:780 fetch_folder_grants awaits its per-folder get + get_member sequentially, so a doc in K folders costs K× round-trip latency on the access-check path.

Fix

join_all the per-folder fetches. Pairs with #P4 (folder-metadata cache). Latency-only; RCU unchanged.

CRDT impact: none beyond the #P4 WS-auth caveat (this is pure parallelization of existing reads).