Expire presence so users can't get stuck online (#77)

Presence was a per-user refcount in Redis, decremented only when a
connection's cleanup ran. Anything that ended a connection without
cleanup (a crash, an unclean restart, a laptop asleep with the page
frozen) leaked its count forever, so those users never showed offline.
Production had 19 counts for 2 live connections.

Presence is now one expiring entry per connection (90s TTL) for global,
per-room and focus presence, in new presence:v2:* keys. Clients renew
theirs by sending any frame, which the existing 30s ping/pong already
does. The server closes a human connection that sends nothing for 90s
(code 4408), which catches a frozen page whose browser still answers
transport pings. Bot connections can't be expected to answer pings, so
the server renews theirs itself while the handler is alive.

An expiry has no disconnect event to announce it, so a sweeper on each
worker (one per 15s overall, via a Redis lock) prunes expired entries
and broadcasts the offline transition. The old presence:* hashes are
deleted at startup.

Tests cover the expiry store, the sweeper, and the websocket cases.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

backend/tests/test_expiring_presence.py

@@ -0,0 +1,155 @@
+import os
+import time
+import uuid
+
+import pytest_asyncio
+from redis.asyncio import Redis
+
+from app.ws.expiring_presence import ExpiringPresenceSet
+
+
+class Clock:
+    def __init__(self) -> None:
+        self.now = time.time()
+
+    def __call__(self) -> float:
+        return self.now
+
+    def advance(self, seconds: float) -> None:
+        self.now += seconds
+
+
+@pytest_asyncio.fixture
+async def redis_client():
+    client = Redis.from_url(os.environ["REDIS_URL"], decode_responses=True)
+    yield client
+    await client.aclose()
+
+
+@pytest_asyncio.fixture
+async def setup(redis_client):
+    clock = Clock()
+    store = ExpiringPresenceSet(redis_client, ttl_seconds=90, clock=clock)
+    key = f"presence:v2:test:{uuid.uuid4()}"
+    yield store, clock, key
+    await redis_client.delete(key)
+
+
+async def test_first_connection_is_a_transition_and_a_second_is_not(setup):
+    store, _, key = setup
+    user = uuid.uuid4()
+    assert await store.add(key, user, "tab1") is True
+    assert await store.add(key, user, "tab2") is False
+    assert await store.live_user_ids(key) == {user}
+
+
+async def test_renewing_a_live_entry_is_not_a_transition(setup):
+    store, clock, key = setup
+    user = uuid.uuid4()
+    await store.add(key, user, "tab1")
+    clock.advance(30)
+    assert await store.add(key, user, "tab1") is False
+
+
+async def test_entry_stops_counting_after_the_ttl(setup):
+    store, clock, key = setup
+    user = uuid.uuid4()
+    await store.add(key, user, "tab1")
+    clock.advance(89)
+    assert await store.is_live(key, user) is True
+    clock.advance(2)
+    assert await store.is_live(key, user) is False
+    assert await store.live_user_ids(key) == set()
+
+
+async def test_renewal_keeps_a_connection_alive_past_the_original_expiry(setup):
+    store, clock, key = setup
+    user = uuid.uuid4()
+    await store.add(key, user, "tab1")
+    for _ in range(5):
+        clock.advance(60)
+        await store.add(key, user, "tab1")
+    assert await store.is_live(key, user) is True
+
+
+async def test_coming_back_after_expiry_is_a_transition_again(setup):
+    store, clock, key = setup
+    user = uuid.uuid4()
+    await store.add(key, user, "tab1")
+    clock.advance(200)
+    assert await store.add(key, user, "tab1") is True
+
+
+async def test_one_live_connection_keeps_the_user_online_when_another_expires(setup):
+    store, clock, key = setup
+    user = uuid.uuid4()
+    await store.add(key, user, "old-tab")
+    clock.advance(60)
+    await store.add(key, user, "new-tab")
+    clock.advance(60)  # old-tab is now 120s stale, new-tab 60s
+    assert await store.is_live(key, user) is True
+
+
+async def test_remove_reports_only_the_last_connection_leaving(setup):
+    store, _, key = setup
+    user = uuid.uuid4()
+    await store.add(key, user, "tab1")
+    await store.add(key, user, "tab2")
+    assert await store.remove(key, user, "tab1") is False
+    assert await store.remove(key, user, "tab2") is True
+
+
+async def test_remove_of_an_entry_that_is_already_gone_reports_nothing(setup):
+    # Whoever removed it first (e.g. prune) already reported the transition.
+    store, _, key = setup
+    user = uuid.uuid4()
+    await store.add(key, user, "tab1")
+    await store.remove(key, user, "tab1")
+    assert await store.remove(key, user, "tab1") is False
+
+
+async def test_remove_of_an_expired_but_unpruned_entry_still_reports_offline(setup):
+    # Nobody has been told this user went offline yet (no prune has run), so
+    # their eventual clean close is the one place left to report it.
+    store, clock, key = setup
+    user = uuid.uuid4()
+    await store.add(key, user, "tab1")
+    clock.advance(200)
+    assert await store.remove(key, user, "tab1") is True
+
+
+async def test_users_are_independent(setup):
+    store, _, key = setup
+    alice, bob = uuid.uuid4(), uuid.uuid4()
+    await store.add(key, alice, "a")
+    await store.add(key, bob, "b")
+    await store.remove(key, alice, "a")
+    assert await store.live_user_ids(key) == {bob}
+
+
+async def test_prune_reports_users_who_lost_their_last_live_entry(setup):
+    store, clock, key = setup
+    gone, still_here = uuid.uuid4(), uuid.uuid4()
+    await store.add(key, gone, "tab1")
+    await store.add(key, still_here, "old")
+    clock.advance(60)
+    await store.add(key, still_here, "new")
+    clock.advance(40)  # gone/tab1 and still_here/old are expired; still_here/new is not
+    assert await store.prune(key) == {gone}
+
+
+async def test_prune_is_idempotent_and_removes_what_it_reports(setup):
+    store, clock, key = setup
+    user = uuid.uuid4()
+    await store.add(key, user, "tab1")
+    clock.advance(200)
+    assert await store.prune(key) == {user}
+    assert await store.prune(key) == set()
+
+
+async def test_prune_with_nothing_expired_does_nothing(setup):
+    store, _, key = setup
+    user = uuid.uuid4()
+    await store.add(key, user, "tab1")
+    assert await store.prune(key) == set()
+    assert await store.is_live(key, user) is True