fix: publish NIP-52 events with monotonic created_at (#26)

NIP-52 calendar events (31922/31923) are replaceable and republished
whenever inventory changes (a ticket sells). build_nip52_event stamped
created_at=int(time.time()); relays only push a replacement to OPEN
subscriptions when created_at is strictly newer, so two republishes in
the same wall-clock second tie and the second is silently dropped for
live subscribers — clients' "tickets remaining" badge stalls until a
reload. Same root cause as the webapp fix (aiolabs/webapp#122).

- Add monotonic_created_at() in nostr_timestamp.py = max(now, last+1),
  mirroring the webapp helper + docs/nostr-patterns/replaceable-events.md.
- Anchor it on the already-persisted Event.nostr_event_created_at
  (set after each publish in nostr_hooks.py). The kind-5 delete event is
  not replaceable, so it keeps plain int(time.time()).
- Unit tests mirror the webapp's timestamp suite.

Concurrent same-second sales reading the same stored anchor can still
collide; full hardening (row-level lock) is noted as follow-up in #26.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_nostr_timestamp.py

@@ -0,0 +1,32 @@
+from itertools import pairwise
+
+from ..nostr_timestamp import monotonic_created_at
+
+
+def test_no_prior_uses_now():
+    assert monotonic_created_at(None, now=1000) == 1000
+
+
+def test_same_second_bumps_past_prior():
+    # now == last: a naive int(time.time()) would tie and the relay would
+    # drop the update; we must produce a strictly newer stamp.
+    assert monotonic_created_at(1000, now=1000) == 1001
+
+
+def test_tracks_wallclock_once_seconds_elapse():
+    assert monotonic_created_at(1000, now=1005) == 1005
+
+
+def test_steps_past_future_dated_prior():
+    # clock skew / rapid bursts left the stored value ahead of now
+    assert monotonic_created_at(2000, now=1000) == 2001
+
+
+def test_strictly_increasing_same_second_burst():
+    last = None
+    stamps = []
+    for _ in range(5):
+        last = monotonic_created_at(last, now=1000)  # clock frozen at 1000
+        stamps.append(last)
+    assert stamps == [1000, 1001, 1002, 1003, 1004]
+    assert all(b > a for a, b in pairwise(stamps))