feat(classes): pure capacity/waitlist logic + Zod schemas

classes-logic.ts: effectiveCapacity (session override > class > unlimited),
computeSignupStatus (registered vs waitlisted), sessionTiming, canSignUp
eligibility, pickPromotion (earliest waitlisted when a seat frees).
Dependency-free, 10 unit tests (suite 372 -> 382).

validations.ts: class/session/signup/attendance schemas. payment_link is
a generic http(s) link only (empty -> null); capacity is a positive int;
no live payment integration.

lib/classes-logic.ts

@@ -0,0 +1,99 @@
+// Pure, dependency-free decision logic for the classes feature. No Supabase,
+// no React, no Next. Unit-tested directly so capacity/waitlist/eligibility
+// rules cannot silently drift.
+
+export type SignupOutcome = 'registered' | 'waitlisted'
+export type SessionTiming = 'upcoming' | 'in_progress' | 'past'
+
+// A session-level capacity overrides the class default; null at both levels
+// means unlimited.
+export function effectiveCapacity(
+  sessionCapacity: number | null | undefined,
+  classCapacity: number | null | undefined,
+): number | null {
+  if (sessionCapacity != null) return sessionCapacity
+  if (classCapacity != null) return classCapacity
+  return null
+}
+
+// Where a new signup lands given the effective capacity and how many are
+// already registered (not counting waitlisted/cancelled).
+export function computeSignupStatus(
+  capacity: number | null,
+  currentRegistered: number,
+): SignupOutcome {
+  if (capacity == null) return 'registered'
+  return currentRegistered < capacity ? 'registered' : 'waitlisted'
+}
+
+function ms(value: string | null | undefined): number | null {
+  if (!value) return null
+  const t = new Date(value).getTime()
+  return Number.isNaN(t) ? null : t
+}
+
+export function sessionTiming(
+  startsAt: string,
+  endsAt: string | null | undefined,
+  now?: Date | string,
+): SessionTiming {
+  const t = now == null ? Date.now() : new Date(now).getTime()
+  const start = ms(startsAt)
+  if (start == null) return 'upcoming'
+  const end = ms(endsAt) ?? start
+  if (t < start) return 'upcoming'
+  if (t > end) return 'past'
+  return 'in_progress'
+}
+
+export type SignupEligibility = { ok: true } | { ok: false; reason: string }
+
+// Whether a member may newly sign up for a session.
+export function canSignUp(input: {
+  sessionStatus: string
+  startsAt: string
+  endsAt?: string | null
+  now?: Date | string
+}): SignupEligibility {
+  if (input.sessionStatus === 'cancelled') return { ok: false, reason: 'This session was cancelled.' }
+  if (input.sessionStatus === 'completed') return { ok: false, reason: 'This session is already completed.' }
+  if (sessionTiming(input.startsAt, input.endsAt, input.now) === 'past') {
+    return { ok: false, reason: 'This session has already ended.' }
+  }
+  return { ok: true }
+}
+
+// When a registered member cancels and a seat frees, the earliest-signed-up
+// waitlisted member is promoted. Returns that signup id, or null if there is
+// no free seat or no one waiting.
+export function pickPromotion(
+  signups: Array<{ id: string; status: string; signed_up_at: string }>,
+  capacity: number | null,
+): string | null {
+  if (capacity == null) {
+    // Unlimited: anyone waitlisted (shouldn't normally happen) can move up;
+    // promote the earliest to be safe.
+    const waiting = signups
+      .filter(s => s.status === 'waitlisted')
+      .sort((a, b) => a.signed_up_at.localeCompare(b.signed_up_at))
+    return waiting[0]?.id ?? null
+  }
+  const registered = signups.filter(s => s.status === 'registered').length
+  if (registered >= capacity) return null
+  const waiting = signups
+    .filter(s => s.status === 'waitlisted')
+    .sort((a, b) => a.signed_up_at.localeCompare(b.signed_up_at))
+  return waiting[0]?.id ?? null
+}
+
+export const SESSION_STATUS_LABEL: Record<string, string> = {
+  scheduled: 'Scheduled',
+  cancelled: 'Cancelled',
+  completed: 'Completed',
+}
+
+export const SIGNUP_STATUS_LABEL: Record<string, string> = {
+  registered: 'Registered',
+  waitlisted: 'Waitlisted',
+  cancelled: 'Cancelled',
+}