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/validations.ts

@@ -741,6 +741,94 @@ export const listMemberCertificationsSchema = z.object({
   memberId: z.string().uuid('Invalid member ID'),
 })
 
+// ─── Classes ─────────────────────────────────────────────────────────────────
+
+// Generic manual payment link only (no live payment integration). Empty
+// string -> null so a cleared field is stored as "no link".
+const paymentLink = z
+  .string()
+  .trim()
+  .max(2000)
+  .optional()
+  .nullable()
+  .transform(v => (v && v !== '' ? v : null))
+  .refine(
+    v => v === null || /^https?:\/\/.+/i.test(v),
+    'Payment link must be an http(s) URL',
+  )
+
+const classCapacity = z
+  .number()
+  .int('Capacity must be a whole number')
+  .positive('Capacity must be at least 1')
+  .max(100000)
+  .optional()
+  .nullable()
+
+const dateTimeString = z.string().min(1, 'A date and time is required').max(40)
+
+export const createClassSchema = z.object({
+  title: z.string().min(1, 'Title is required').max(200),
+  description: z.string().max(5000).optional().nullable(),
+  payment_link: paymentLink,
+  capacity: classCapacity,
+  grants_certification_id: z.string().uuid('Invalid certification ID').optional().nullable(),
+})
+
+export const updateClassSchema = z.object({
+  classId: z.string().uuid('Invalid class ID'),
+  title: z.string().min(1).max(200).optional(),
+  description: z.string().max(5000).optional().nullable(),
+  payment_link: paymentLink,
+  capacity: classCapacity,
+  grants_certification_id: z.string().uuid('Invalid certification ID').optional().nullable(),
+  is_active: z.boolean().optional(),
+})
+
+export const classIdSchema = z.object({
+  classId: z.string().uuid('Invalid class ID'),
+})
+
+export const createSessionSchema = z.object({
+  classId: z.string().uuid('Invalid class ID'),
+  starts_at: dateTimeString,
+  ends_at: z.string().max(40).optional().nullable().transform(v => (v && v !== '' ? v : null)),
+  location: z.string().max(300).optional().nullable(),
+  capacity: classCapacity,
+  notes: z.string().max(5000).optional().nullable(),
+})
+
+export const updateSessionSchema = z.object({
+  sessionId: z.string().uuid('Invalid session ID'),
+  starts_at: dateTimeString.optional(),
+  ends_at: z.string().max(40).optional().nullable().transform(v => (v && v !== '' ? v : null)),
+  location: z.string().max(300).optional().nullable(),
+  capacity: classCapacity,
+  notes: z.string().max(5000).optional().nullable(),
+  status: z.enum(['scheduled', 'cancelled', 'completed']).optional(),
+})
+
+export const sessionIdSchema = z.object({
+  sessionId: z.string().uuid('Invalid session ID'),
+})
+
+export const signUpForClassSchema = z.object({
+  sessionId: z.string().uuid('Invalid session ID'),
+})
+
+export const cancelSignupSchema = z.object({
+  sessionId: z.string().uuid('Invalid session ID'),
+})
+
+export const markAttendanceSchema = z.object({
+  signupId: z.string().uuid('Invalid signup ID'),
+  attended: z.boolean(),
+})
+
+export const listSessionSignupsSchema = z.object({
+  sessionId: z.string().uuid('Invalid session ID'),
+})
+
 // ─── Generic ID schemas ──────────────────────────────────────────────────────
 
 export const uuidSchema = z.string().uuid('Invalid ID format')