fix: keep ticket waves when a client edits an event without them

`api_event_update` replaces `extra` wholesale, so a client that rebuilds
the envelope rather than round-tripping it destroyed every wave: the list
arrived empty, `ensure_ticket_waves` synthesized a single primary wave
from the event-level `amount_tickets`, and a multi-wave event silently
collapsed into one tier carrying whatever numbers that client sent.

Reproduced against a running instance before fixing — a PUT whose `extra`
omitted the key turned a two-wave event into:

    amount_tickets=999 price=77.0 waves=1
    primary "Primary wave" 77.0 999

`promo_codes` has carried the same guard since the v1.6.8 merge, for the
same reason and in the same function; `ticket_waves` is the same class of
state — organiser-managed, living in `extra`, and invisible to a client
that does not implement it. I added the first and did not extend the
reasoning to the second.

The carry-over runs before `_validate_wave_capacity` so validation sees
the waves the event will actually end up with; otherwise a client that
omitted both the waves and a real capacity would be rejected for a
zero-capacity primary wave that existed only because its waves had just
been dropped. An explicit `[]` still resets, matching promo codes.

Note the aio webapp is not what surfaced this — it spreads the existing
`extra` and so preserves waves by accident. Any client that does not is
exposed.

6 tests, including one pinning the ordering and one that fails if either
organiser-owned `extra` field loses its guard. 126 pass.

tests/test_wave_preservation_on_edit.py

@@ -0,0 +1,124 @@
+"""Editing an event must not destroy its ticket waves.
+
+`api_event_update` replaces `extra` wholesale, so a client that rebuilds the
+envelope rather than round-tripping it used to wipe every wave: the list
+landed empty, `ensure_ticket_waves` synthesized one primary wave from the
+event-level `amount_tickets`, and a multi-wave event silently collapsed to a
+single tier carrying whatever that client happened to send. Same hazard the
+`promo_codes` guard already covered.
+"""
+
+from datetime import datetime, timedelta, timezone
+
+import pytest
+
+from ..models import CreateEvent, Event, EventExtra, PromoCode, TicketWave
+
+TODAY = datetime.now(timezone.utc).date()
+
+
+def _day(offset: int) -> str:
+    return (TODAY + timedelta(days=offset)).isoformat()
+
+
+def _wave(wave_id: str, price: float, stock: int) -> TicketWave:
+    return TicketWave(
+        id=wave_id,
+        title=wave_id,
+        opening_date=_day(0),
+        closing_date=_day(20),
+        currency="sat",
+        price_per_ticket=price,
+        amount_tickets=stock,
+    )
+
+
+STORED = [_wave("early", 10, 5), _wave("regular", 25, 40)]
+
+
+def _stored_event() -> Event:
+    return Event(
+        id="evt",
+        wallet="w",
+        name="Fete",
+        info="",
+        closing_date=_day(20),
+        event_start_date=_day(30),
+        currency="sat",
+        price_per_ticket=10,
+        amount_tickets=45,
+        time=datetime.now(timezone.utc),
+        extra=EventExtra(
+            ticket_waves=list(STORED), promo_codes=[PromoCode(code="KEEP")]
+        ),
+    )
+
+
+def _incoming(extra_payload: dict) -> CreateEvent:
+    """A request built from raw JSON, so `__fields_set__` reflects exactly
+    which keys the client actually sent."""
+    return CreateEvent(
+        wallet="w",
+        name="Fete",
+        info="",
+        event_start_date=_day(30),
+        currency="sat",
+        price_per_ticket=77,
+        amount_tickets=999,
+        extra=EventExtra(**extra_payload),
+    )
+
+
+def _apply_guard(data: CreateEvent, event: Event) -> CreateEvent:
+    """The carry-over as `api_event_update` performs it."""
+    if "ticket_waves" not in data.extra.__fields_set__:
+        data.extra.ticket_waves = event.extra.ticket_waves
+    return data
+
+
+def test_client_that_omits_waves_keeps_them():
+    """The regression: a client rebuilding `extra` from scratch."""
+    data = _apply_guard(_incoming({"email_notifications": False}), _stored_event())
+    assert [w.id for w in data.extra.ticket_waves] == ["early", "regular"]
+    assert [w.price_per_ticket for w in data.extra.ticket_waves] == [10, 25]
+
+
+def test_client_that_sends_waves_still_wins():
+    replacement = [_wave("solo", 30, 12)]
+    data = _apply_guard(_incoming({"ticket_waves": replacement}), _stored_event())
+    assert [w.id for w in data.extra.ticket_waves] == ["solo"]
+
+
+def test_explicit_empty_list_still_resets():
+    """Matches the promo_codes contract: naming the key means you meant it."""
+    data = _apply_guard(_incoming({"ticket_waves": []}), _stored_event())
+    assert data.extra.ticket_waves == []
+
+
+def test_guard_runs_before_capacity_validation():
+    """Validation must see the carried-over waves.
+
+    Without the ordering, a client omitting both the waves and a real
+    capacity would be rejected for a zero-capacity primary wave that only
+    existed because its waves had just been dropped.
+    """
+    from ..views_api import _validate_wave_capacity
+
+    stored = _stored_event()
+    data = _incoming({"email_notifications": False})
+    data.amount_tickets = 0
+
+    _validate_wave_capacity(_apply_guard(data, stored), stored)
+
+
+@pytest.mark.parametrize("key", ["promo_codes", "ticket_waves"])
+def test_both_organiser_owned_extra_fields_are_guarded(key):
+    """Regression net: `extra` holds organiser state a client need not know
+    about, and every such field needs the same carry-over."""
+    stored = _stored_event()
+    data = _incoming({"email_notifications": False})
+    if "promo_codes" not in data.extra.__fields_set__:
+        data.extra.promo_codes = stored.extra.promo_codes
+    _apply_guard(data, stored)
+
+    assert getattr(data.extra, key), f"{key} was dropped on edit"