Inspired by the quote lifecycle work in #1022 (wallet-side handling of unpaid/expired invoices), this PR hardens the mint side: websocket subscriptions to bolt11_mint_quote are closed when all subscribed quotes are unpaid and past expiry, instead of holding connections open until mint_websocket_read_timeout (default 600s).

Also fixes mint quote expiry not being persisted or returned on GET: POST returned expiry, but GET and the websocket monitor saw null because mint_quotes had no expiry column and store_mint_quote never wrote it.

Problem

• WebSocket clients subscribing to an unpaid mint quote could keep a connection open until the read timeout (600s) after the quote TTL, even though the quote is no longer actionable.
• expiry was computed at quote creation but not stored in mint_quotes (unlike melt_quotes), so REST GET, MintQuote.from_row, and the expiry monitor could not see it.

Changes

• MintQuote.from_row: load expiry from DB rows (SQLite + Postgres).
• store_mint_quote: persist expiry on insert.
• Migration m036_add_expiry_to_mint_quotes: add expiry column to mint_quotes.
• LedgerEventClientManager: background poll (MINT_WEBSOCKET_QUOTE_EXPIRY_CHECK_INTERVAL, default 30s) closes WS with code 1000 / reason mint quote subscription expired when every subscribed mint quote is unpaid and expiry <= now.
• Tests: websocket monitor unit tests + assert TTL quote survives round-trip via get_mint_quote.

Verification

Local mint:

MINT_QUOTE_TTL=60
FAKEWALLET_BRR=false
MINT_WEBSOCKET_QUOTE_EXPIRY_CHECK_INTERVAL=5

export MINT_URL=http://127.0.0.1:3338
QUOTE_RESP=$(curl -sS -X POST "$MINT_URL/v1/mint/quote/bolt11" \
  -H 'Content-Type: application/json' -d '{"amount": 64, "unit": "sat"}')
QUOTE_ID=$(echo "$QUOTE_RESP" | jq -r .quote)

POST and GET both return matching expiry:

{ "quote": "k4D70Q30zKmxgnAGmxGcGLYSdRNKL71TXj8ZRttb", "state": "UNPAID", "expiry": 1780484602 }

WebSocket (python3 /tmp/ws_quote_hold.py "$QUOTE_ID"), ~60s after quote creation:

CLOSED 1000 mint quote subscription expired

Mint log:

Closing websocket: all subscribed mint quotes expired unpaid

After expiry, GET still returns state: "UNPAID" with expiry set expected: state tracks payment/mint progress; expiry is a separate deadline. Clients treat UNPAID + expiry < now as dead; POST /v1/mint/bolt11 rejects with quote expired.

Test plan

 pytest tests/mint/test_mint_websocket_protocol.py

 pytest tests/mint/test_mint.py::test_mint_quote_ttl_setting_overrides_invoice_expiry

 Manual repro above (POST/GET expiry match, WS close at TTL + poll interval)