fix(ecash): give every node a backup phrase, and prove restore works

Running the route suite on this box surfaced that the backup was
unreachable here: `identity/master_seed.enc` is written during
onboarding, and any node onboarded before that step existed simply does
not have one. Reveal bailed with "this node has no encrypted seed
backup", and restore followed it down.

But the choice on such a node was never "derived phrase or independent
phrase" — it was "independent phrase or no backup at all", and a wallet
whose coins can be restored from words the operator holds beats one
whose coins die with a single file. So it now generates one, recorded as
`independent`, and every surface that shows it says plainly that
restoring the node will not bring the ecash back — only these words
will. `derivable_from_node_seed` lets the card say which kind you are
about to get *before* you write anything down.

Also: a mint that never implemented NUT-09 answered restore with a bare
404, which surfaced as "mint returned 404 with no further detail" —
true, and useless to someone trying to get their coins back. It now
names the limitation.

The route suite was reading `result.amount_sats` from mint-claim, which
answers with `minted_sats`. A working claim had been reporting as a
failure; that was one of the two reds carried over from yesterday.

The real gap, though, was that "recovered 0 sats" passes on a wallet
with nothing to find — exactly the shape of a backup that looks fine
until the day you need it. test-ecash-restore.sh does the test that
settles it: mint, **delete the wallet file**, restore, check the coins
came back. On this box: 87 sats before the wipe, 0 after, 61 recovered
from the phrase alone — every coin minted since the phrase existed, and
none of the 26 sats minted before it, which used random secrets and
never could come back. Testnet only, and it refuses to run otherwise.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

scripts/test-ecash-routes.sh

@@ -140,7 +140,9 @@ fi
 
 if [ -n "$QUOTE" ]; then
   res="$(rpc wallet.ecash-mint-claim "{\"quote_id\":\"$QUOTE\",\"amount_sats\":16}")"
-  claimed="$(printf '%s' "$res" | jqf result.amount_sats)"
+  # The handler answers with `minted_sats`; reading `amount_sats` here made a
+  # working claim look like a failure (it was one of the two reds on 2026-08-17).
+  claimed="$(printf '%s' "$res" | jqf result.minted_sats)"
   if [ -n "$claimed" ]; then
     ok "ecash-mint-claim minted ${claimed} sats"
   elif [ "$NETWORK" = mainnet ]; then