mintdemux: melt the new half of a mixed selection instead of refusing

A wallet holding leftovers of both eras selects proofs from each, and the two
mints cannot co-sign one melt, so such wallets were stuck for good behind the
"melt them separately" error. Drop the old inputs and melt the new ones. The
old proofs stay unspent at the old mint while the wallet treats them as spent,
so each one is recorded with its keyset, amount, secret hash and whether it
carried DLEQ; they were only redeemable through the recovery path anyway.

Also stop the routing tables from going empty. The hourly refresh is exempt
from the mints' per-ip limiter, so it now seeds the last-good keyset lists the
client-facing /v1/keysets falls back on, and a start that races the mints
coming up retries every 15s rather than waiting out the hour -- an empty old
list makes every old-ecash melt look new and routes it to the wrong mint.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

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