fix(tickets): amount_tickets is the remaining count, stop subtracting sold

`set_ticket_paid` decrements `amount_tickets` on every sale and
increments `sold`, so the two describe the same tickets. Subtracting
one from the other in `api_ticket_create` removed each sale twice:

  remaining = amount_tickets - sold

That under-reported availability to buyers, and because
`remaining + sold` is the original capacity, `sold >= amount_tickets`
first becomes true at the halfway point — so every event locked itself
as sold out once half its seats had gone, with the rest still unsold.

Measured on aio-demo before the fix: 16 of 24 live events affected,
3 of them already refusing sales while stock remained. "Tech Meetup"
had 9 tickets left and offered -2.

The arithmetic was ours, introduced with the multi-quantity purchase
feature; upstream has no equivalent because it sells one ticket per
request and reads `amount_tickets` as remaining everywhere. So this
deletes the divergence and restores upstream's own guard plus the one
line `quantity` needs, which should also make the v1.6.8 rebase (#33)
a little easier rather than harder.

Tests walk a 50-seat event to capacity one sale at a time and pin the
boundary: sold=49 passes even unfixed, sold=50 is where it used to
lock. Six of the ten fail without the change.

Scoped deliberately to the double-subtraction. The rest of #34 — making
capacity a required field, dropping the `0 = unlimited` reading, and the
organizer form that prefills remaining as though it were capacity —
waits for #33, since ticket waves change the shape of that fix.

Refs #34

views_api.py

@@ -670,17 +670,18 @@ async def api_ticket_create(
     if event.canceled:
         raise HTTPException(status_code=HTTPStatus.GONE, detail="Event is canceled.")
     quantity = data.quantity
-    if event.amount_tickets > 0:
-        if event.sold >= event.amount_tickets:
-            raise HTTPException(
-                status_code=HTTPStatus.GONE, detail="Event is sold out."
-            )
-        remaining = event.amount_tickets - event.sold
-        if quantity > remaining:
-            raise HTTPException(
-                status_code=HTTPStatus.BAD_REQUEST,
-                detail=f"Only {remaining} ticket(s) remaining for this event.",
-            )
+    # `amount_tickets` IS the remaining count — `set_ticket_paid` decrements
+    # it on every sale, and upstream reads it the same way everywhere. Do
+    # not subtract `sold` from it: `sold` counts the same tickets the
+    # decrement already removed, so doing both takes each sale off twice
+    # (aiolabs/events#34).
+    if event.amount_tickets < 1:
+        raise HTTPException(status_code=HTTPStatus.GONE, detail="Event is sold out.")
+    if quantity > event.amount_tickets:
+        raise HTTPException(
+            status_code=HTTPStatus.BAD_REQUEST,
+            detail=f"Only {event.amount_tickets} ticket(s) remaining for this event.",
+        )
 
     name = data.name
     email = data.email