fix: require a capacity on every ticket wave

"Capacity is always required, there is no unlimited" was settled on #34
and enforced in #62 — but as a single number on the event. Since v1.6.8
capacity is per-wave and `event.amount_tickets` is a derived roll-up
(`sync_event_ticket_waves`), so the rule had nothing holding it up at
the level organisers actually set: the wave form's capacity input had no
minimum, and nothing on the backend checked one at all.

A zero-capacity wave can never be active — `get_active_ticket_waves`
requires `amount_tickets > 0` — so it is the wave-level form of exactly
what #62 removed: an event that looks on sale but refuses every
purchase, on the card and at checkout both.

`_validate_wave_capacity` now runs on create and update. Two details
that matter:

- it checks `ensure_ticket_waves(data)` rather than the raw list, so an
  event submitted with no waves is checked through the primary wave it
  is about to be given, not vacuously passed
- on an edit it only checks waves NEW to the event. Selling out is the
  legitimate route to zero, and rejecting it would make a sold-out event
  uneditable — including the legacy zero-capacity rows this rule exists
  to let organisers fix

Frontend: the wave dialog's capacity input gains the `min="1"` and hint
the event-level field already had, and a new wave opens at 1 rather than
0. That default uses `??`, not `||` — a sold-out wave holds 0 and must
keep showing it instead of silently regaining stock when saved.

Also drops the stale `0 = unlimited / not ticketed` contract still
documented on `CreateEvent.amount_tickets`, which has not been true
since #62.

6 new tests; 120 pass. ruff, black, prettier clean; mypy error set
unchanged from baseline.

models.py

@@ -115,7 +115,12 @@ class CreateEvent(BaseModel):
     currency: str = "sat"
     allow_fiat: bool = False
     fiat_currency: str = "GBP"
-    amount_tickets: int = 0  # 0 = unlimited / not ticketed
+    # Capacity is always required and there is no unlimited (#34): a zero
+    # here means sold out / not sellable, which `api_get_event` and
+    # `api_ticket_create` both enforce with a 410. Under v1.6.8 waves this
+    # is only the seed for the primary wave — `sync_event_ticket_waves`
+    # recomputes it as the sum of every wave's remaining stock.
+    amount_tickets: int = 0
     price_per_ticket: float = 0  # 0 = free
     banner: str | None = None
     location: str | None = None  # venue/address (NIP-52 'location' tag)