feat: multi-ticket purchases as N rows sharing one payment_hash

Replaces the previous "one row, N seats via extra.quantity" model
with proper one-row-per-attendee semantics. Each attendee gets a
unique scannable id; the door PUT /register/{ticket_id} marks
them registered independently — so a buyer can purchase 3 tickets,
hand 2 QRs to friends arriving separately, and each attendee can
enter on their own schedule.

Schema (migrations_fork.py m002):
- ticket.payment_hash: new TEXT column shared across all rows of
  a multi-ticket purchase. Backfilled `payment_hash = id` for
  pre-migration rows (id WAS the payment_hash by invariant).

Wire:
- TicketPaymentRequest grows `ticket_ids: list[str]` so the
  webapp gets every scannable id back in the create response.
- POST /tickets/{event_id}/{payment_hash} polling endpoint now
  reports `ticket_ids` (every row) + keeps `ticket_id` for
  back-compat.
- api_ticket_create loops quantity times; the first row reuses
  payment_hash as id (preserves legacy `id == payment_hash`
  invariant for single-ticket purchases), the rest get
  urlsafe_short_hash() uuids.

Payment flow:
- on_invoice_paid fetches all rows by payment_hash and marks each
  paid via set_ticket_paid, which now increments event.sold by 1
  per row (was N per row via extra.quantity — simpler now). The
  per-event asyncio lock still serializes counter + republish so
  concurrent multi-ticket purchases for the same event don't
  reorder the published Nostr state.
- Each paid row triggers its own send_ticket_notification_in_
  background call — no-op for buyers without nostr_identifier /
  email, useful when the buyer set those on the row.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

models.py

@@ -133,6 +133,9 @@ class CreateTicket(BaseModel):
     nostr_identifier: str | None = None
     payment_method: str | None = None
     fiat_provider: str | None = None
+    # Number of tickets to buy on this single invoice. Bounded so a
+    # bad client can't run away with the organizer's capacity.
+    quantity: int = Field(default=1, ge=1, le=10)
 
     @root_validator
     def validate_identifiers(cls, values):
@@ -158,6 +161,11 @@ class Ticket(BaseModel):
     time: datetime
     reg_timestamp: datetime
     extra: TicketExtra = Field(default_factory=TicketExtra)
+    # Shared LNbits invoice payment_hash. Equals `id` for single-ticket
+    # purchases (legacy + post-migration default). Multi-ticket
+    # purchases create N rows sharing one payment_hash so each attendee
+    # gets a distinct scannable id while the buyer pays once.
+    payment_hash: str | None = None
 
 
 class PublicTicket(BaseModel):
@@ -175,3 +183,8 @@ class TicketPaymentRequest(BaseModel):
     fiat_payment_request: str | None = None
     fiat_provider: str | None = None
     is_fiat: bool = False
+    # Row ids created on this invoice — one for single-ticket
+    # purchases, N for multi-ticket (each independently scannable at
+    # the door). Buyers fetch these after payment to render N QRs in
+    # My Tickets.
+    ticket_ids: list[str] = Field(default_factory=list)