Add Tickets domain for the voucher-based ticket shop (#8)

Reviewed-on: #8
Co-authored-by: Patrick Mueller <patrick@mueller-patrick.tech>
Co-committed-by: Patrick Mueller <patrick@mueller-patrick.tech>

src/models/tickets/tickets.capacity.ts

@@ -0,0 +1,41 @@
+export interface EventTicketState {
+	eventId: number;
+	capacity: number | null;
+	redemptionDeadline: Date | null;
+	collectAddress: boolean;
+	requireAddress: boolean;
+	guestsUsed: number;
+	spotsRemaining: number | null;
+}
+
+/**
+ * Reads an event's voucher settings + live guest count within the caller's
+ * connection/transaction. Absence of a settings row means uncapped/no
+ * deadline/no address collection - the "absence over sentinels" convention
+ * also used by the Feedback module.
+ *
+ * Pass forUpdate=true from inside the redeem transaction to lock the
+ * settings row for the duration of that transaction, serializing concurrent
+ * redemptions against the same event so the guestsUsed sum computed here
+ * stays correct even under a last-spot race. Events with no settings row
+ * (uncapped) don't need this - there's no cap to race against.
+ */
+export const getEventTicketState = async (conn: any, eventId: number, forUpdate = false): Promise<EventTicketState> => {
+	const settingsQuery = `SELECT capacity, redemption_deadline, collect_address, require_address FROM event_ticket_settings WHERE event_id = ?${forUpdate ? ' FOR UPDATE' : ''}`;
+	const settingsRows = await conn.query(settingsQuery, [eventId]);
+	const capacity = settingsRows.length > 0 ? settingsRows[0].capacity : null;
+	const redemptionDeadline = settingsRows.length > 0 ? settingsRows[0].redemption_deadline : null;
+	const collectAddress = settingsRows.length > 0 ? !!settingsRows[0].collect_address : false;
+	// Only meaningful when collectAddress is also true - the field isn't
+	// shown/collected at all otherwise, so "required" is moot.
+	const requireAddress = collectAddress && settingsRows.length > 0 ? !!settingsRows[0].require_address : false;
+
+	const usedRows = await conn.query(
+		"SELECT COALESCE(SUM(guest_count), 0) as used FROM redemptions WHERE event_id = ? AND status = 'ACTIVE'",
+		[eventId]
+	);
+	const guestsUsed = Number(usedRows[0].used);
+	const spotsRemaining = capacity === null ? null : Math.max(0, capacity - guestsUsed);
+
+	return {eventId, capacity, redemptionDeadline, collectAddress, requireAddress, guestsUsed, spotsRemaining};
+};