List the requesting user's active allocations. Each entry includes
the on-ledger allocation_cid so clients can drive the
emergency-withdrawal flow.

Optional asset filter narrows results to a single asset. Pass the
user-facing asset name (e.g. CC, USDC); aliasing is handled
server-side. Only allocations with status active are returned. The
expired field is computed server-side: true when
allocation_expiration is set and in the past, false otherwise
(allocations without an expiration never expire).

Returns at most 500 entries; no pagination cursor is needed, as the
number of active allocations per user is bounded.