Codex review on b687922.
The hold derived each new deadline from `Date.now()`, so a caller touching
once a day advanced it by another 24h every time, far below the rate limit.
That left indefinite preservation reachable and made the PR's replay claim
wrong: the window was bounded per call but not in aggregate.
Anchor the deadline to the file's immutable `createdAt` instead of the
request clock. `extendFilesTTL` now takes a lifetime and sets
`expiresAt = max(expiresAt, createdAt + holdMs)` in an aggregation
pipeline, so the target is a fixed point per file and replay is inert
rather than merely bounded. `$max` keeps the widen-only property and the
`expiresAt: {$exists: true}` filter still refuses to resurrect a released
TTL; `createdAt: {$exists: true}` fail-closes when the anchor is absent.
The update runs with `timestamps: false`: a hold is TTL bookkeeping, not a
content write, and bumping `updatedAt` also made every re-touch count as a
modification, hiding whether the deadline actually moved.
Also drop four `.node_modules-*` symlinks that `git add -A` swept in from
an npm install. They pointed at absolute paths on one machine, so every
other checkout got dangling entries. Added the pattern to .gitignore so a
workspace install cannot reintroduce them.
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