fix(mint): close websocket when subscribed mint quotes expire unpaid

[b-l-u-e]

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)

[codecov]

Codecov Report

❌ Patch coverage is 97.22222% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 75.18%. Comparing base (0e9a2dc) to head (a0b6d9f).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
cashu/mint/events/client.py	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1036      +/-   ##
==========================================
+ Coverage   75.13%   75.18%   +0.04%     
==========================================
  Files         110      110              
  Lines       12116    12152      +36     
==========================================
+ Hits         9103     9136      +33     
- Misses       3013     3016       +3     

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[b-l-u-e]

Thank you all for the feedback

made changes now we close WS once all subscribed mint quotes are terminal (PAID or UNPAID && expiry <= now).
Updated monitor logic + tests (including replacing the old “paid keeps open” case).

[callebtc]

I may be not the best idea to enforce restrictions on the websocket as long as we don't restrict anything in the business logic (of minting itself) that depends on the expiry. We don't seem to have consensus on what to do with the expiry yet.

[edgarmuyomba]

I may be not the best idea to enforce restrictions on the websocket as long as we don't restrict anything in the business logic (of minting itself) that depends on the expiry. We don't seem to have consensus on what to do with the expiry yet.

Valid concern — makes sense to wait for consensus on expiry semantics before enforcing them here. Though I think closing connections once all subscribed quotes are paid is worth doing on its own, because a subscribed connection to an already-minted quote has no server-side cleanup mechanism. Also, scoping to just the paid case (for now) would leave the monitor structure in place, making it straightforward to add expiry-based handling once that's settled.