fund: authenticated recovery of a rotated fund's old link

When a fund's secret id has been rotated (invalidating an exfiltrated id list),
its old id 404s. Let a legit owner who bookmarked the old link find the fund
again — but reveal the new id ONLY to an authenticated manager of the new fund,
via a `fund:rotated:<oldid>` -> newid map. An anonymous caller or a non-owner
gets the same "fund not found" as before, so an attacker replaying a leaked old
id learns nothing (an open old->new redirect would defeat the rotation). The
/fund/:id route now runs `optional` auth so req.user is available without
requiring it; bearer funds with no managers are intentionally not recoverable
this way. rotate-funds.ts records the mapping on any future rotation.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01DKkev76U4YRnRAqM6jqbae

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