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>

nostr_timestamp.py

@@ -0,0 +1,34 @@
+"""Monotonic ``created_at`` for replaceable / addressable Nostr events.
+
+Relays only push a replaceable update to OPEN subscriptions when its
+``created_at`` is strictly newer than the version they already hold.
+``created_at`` is integer seconds, so a publisher that stamps
+``int(time.time())`` can emit two versions within the same wall-clock
+second (e.g. two ticket sales republishing the NIP-52 calendar event) —
+the relay treats the second as not-newer and never propagates it to live
+subscribers (it only surfaces on a reload / fresh REQ).
+
+Returning ``max(now, last_created_at + 1)`` guarantees a strictly
+increasing timestamp across successive publishes of the same replaceable
+event. When enough real seconds have elapsed it tracks wall-clock; only
+same-second (or clock-skewed) republishes get nudged forward.
+
+Mirrors the webapp's ``monotonicCreatedAt`` (src/lib/nostr/timestamp.ts)
+and ``docs/nostr-patterns/replaceable-events.md``.
+"""
+
+import time
+
+
+def monotonic_created_at(last_created_at: int | None, now: int | None = None) -> int:
+    """Strictly-newer ``created_at`` for the next publish of a coord.
+
+    :param last_created_at: ``created_at`` of the previously published
+        version (seconds), or ``None`` if none has been published yet.
+    :param now: Current time in seconds — injectable for tests; defaults
+        to ``int(time.time())``.
+    """
+    base = int(time.time()) if now is None else now
+    if last_created_at is None:
+        return base
+    return max(base, last_created_at + 1)