Centralize seat capacity accounting and the payment provider registry.

Replace manualProviders.ts with a paymentProviders.ts registry
(automatic vs manual settlement) and move all seat counting into
capacity.ts as the single source of truth: only paid/checked-in tickets
and pending_approval payments hold a seat, so abandoned checkouts never
block sales. Admins can now knowingly approve a payment over capacity
(allowOverCapacity), with the booking and admin UIs updated to match.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/src/lib/capacity.ts

@@ -0,0 +1,70 @@
+// Single source of truth for event seat accounting.
+//
+// A seat is held by a booking the moment the money is real or claimed to be:
+//   - ticket 'confirmed' or 'checked_in' (paid), or
+//   - ticket 'pending' whose payment is 'pending_approval' (customer clicked
+//     "I've paid" and is waiting for admin verification).
+//
+// A bare 'pending' payment — an opened checkout that was never paid nor claimed,
+// on any provider — holds NO seat, so abandoned bookings can never block sales.
+// The accepted trade-off is a small oversell window when several people book the
+// last seats and all later pay/claim; admin approval is the backstop and may
+// knowingly approve over capacity (see routes/payments.ts).
+//
+// 'pending_approval' is a payment status (the ticket row stays 'pending'), so
+// every capacity count joins tickets to payments. There is exactly one payment
+// row per ticket (created together in routes/tickets.ts).
+//
+// Every place that counts seats — booking creation, public availability,
+// hold recovery, admin dashboards — must go through these builders so the
+// formula cannot diverge between surfaces.
+
+import { sql, and, eq, inArray } from 'drizzle-orm';
+import { tickets, payments } from '../db/index.js';
+
+// COALESCE keeps the predicate two-valued under the LEFT JOIN (a ticket with no
+// payment row must count as "not holding" — NULL would poison NOT ...).
+export const seatHoldingSql = sql`(${(tickets as any).status} IN ('confirmed', 'checked_in') OR (${(tickets as any).status} = 'pending' AND COALESCE(${(payments as any).status}, '') = 'pending_approval'))`;
+
+/**
+ * Query: number of seats currently held for an event.
+ * `executor` is the db, or a transaction (sync sqlite tx: finish with `.get()`;
+ * async pg tx / plain db: await via dbGet).
+ */
+export function seatHolderCountQuery(executor: any, eventId: string) {
+  return executor
+    .select({ count: sql<number>`count(distinct ${(tickets as any).id})` })
+    .from(tickets)
+    .leftJoin(payments, eq((payments as any).ticketId, (tickets as any).id))
+    .where(and(eq((tickets as any).eventId, eventId), seatHoldingSql));
+}
+
+/**
+ * Query: how many of the given tickets do NOT currently hold a seat (and so
+ * would need fresh capacity if promoted to a seat-holding state).
+ */
+export function unseatedTicketCountQuery(executor: any, ticketIds: string[]) {
+  return executor
+    .select({ count: sql<number>`count(distinct ${(tickets as any).id})` })
+    .from(tickets)
+    .leftJoin(payments, eq((payments as any).ticketId, (tickets as any).id))
+    .where(and(inArray((tickets as any).id, ticketIds), sql`NOT ${seatHoldingSql}`));
+}
+
+/**
+ * Query: per-event breakdown of paid vs claimed seats, grouped by event.
+ * paidCount = confirmed + checked_in; claimedCount = pending_approval-held.
+ * Pass `eventId` to restrict to one event (still returns a grouped row).
+ */
+export function eventSeatBreakdownQuery(executor: any, eventId?: string) {
+  const query = executor
+    .select({
+      eventId: (tickets as any).eventId,
+      paidCount: sql<number>`sum(case when ${(tickets as any).status} IN ('confirmed', 'checked_in') then 1 else 0 end)`,
+      claimedCount: sql<number>`sum(case when ${(tickets as any).status} = 'pending' AND COALESCE(${(payments as any).status}, '') = 'pending_approval' then 1 else 0 end)`,
+    })
+    .from(tickets)
+    .leftJoin(payments, eq((payments as any).ticketId, (tickets as any).id));
+  return (eventId ? query.where(eq((tickets as any).eventId, eventId)) : query)
+    .groupBy((tickets as any).eventId);
+}