Add roles and invite models

Additive schema only; no behaviour changes yet. Verified with prisma
migrate diff against production: two enums, three new tables, new User
columns and foreign keys, and no destructive statements.

Role lives on the User row rather than an OWNER_EMAIL env check, because
email is nullable and user-editable and so a poor thing to authorize
against. The jwt callback will need a per-request lookup for status
anyway, so reading role in the same query is free.

Invites are bearer tokens in a URL, shared out of band as a link or QR
code, because the app has no email capability. claimInvite consumes a
use with a single UPDATE guarded on usedCount < maxUses: Prisma cannot
compare two columns in a where clause, and one statement means one row
lock, so two people redeeming the last use cannot both succeed.

SearchCache finally gets its user relation. Every other user-owned model
cascades; without it, deleting a user left orphaned rows holding their
raw search queries. Verified zero orphans before adding the constraint.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lib/invites.ts

@@ -0,0 +1,88 @@
+import { randomBytes } from "crypto"
+import { Prisma } from "@prisma/client"
+import { prisma } from "@/lib/prisma"
+
+/** Cookie carrying the invite token between /invite/<token> and signup. */
+export const INVITE_COOKIE = "dt_invite"
+
+/** How long an invite token stays usable in the browser after the link is opened. */
+export const INVITE_COOKIE_MAX_AGE = 30 * 60
+
+export class InviteError extends Error {
+  constructor(
+    message: string,
+    readonly status: number = 400
+  ) {
+    super(message)
+    this.name = "InviteError"
+  }
+}
+
+/** 128 bits, same shape as the share-list slugs in api/shared-lists/route.ts. */
+export function generateInviteToken(): string {
+  return randomBytes(16).toString("hex")
+}
+
+export function inviteUrl(token: string, origin: string): string {
+  return `${origin.replace(/\/$/, "")}/invite/${token}`
+}
+
+export type InviteState = "valid" | "unknown" | "revoked" | "expired" | "exhausted"
+
+/**
+ * Read-only check, for rendering. Never rely on this to admit a signup - between the
+ * check and the write another request can consume the last use. Use claimInvite for that.
+ */
+export async function inspectInvite(token: string): Promise<{
+  state: InviteState
+  invite?: { id: string; label: string | null; createdById: string; inviterName: string | null }
+}> {
+  if (!token || !/^[a-f0-9]{32}$/.test(token)) return { state: "unknown" }
+
+  const invite = await prisma.invite.findUnique({
+    where: { token },
+    include: { createdBy: { select: { name: true } } },
+  })
+
+  if (!invite) return { state: "unknown" }
+  if (invite.revokedAt) return { state: "revoked" }
+  if (invite.expiresAt && invite.expiresAt <= new Date()) return { state: "expired" }
+  if (invite.usedCount >= invite.maxUses) return { state: "exhausted" }
+
+  return {
+    state: "valid",
+    invite: {
+      id: invite.id,
+      label: invite.label,
+      createdById: invite.createdById,
+      inviterName: invite.createdBy.name,
+    },
+  }
+}
+
+/**
+ * Atomically consume one use of an invite. Returns the invite, or null if it was not
+ * claimable (unknown, revoked, expired, or already exhausted).
+ *
+ * The guard has to be a single UPDATE because Prisma's query builder cannot compare
+ * two columns in a `where` (`usedCount < maxUses`). One statement means one row lock,
+ * so two people redeeming the last use of a link cannot both succeed.
+ *
+ * Call inside a transaction alongside the user creation, so that a later failure
+ * (duplicate email, for instance) rolls the consumed use back.
+ */
+export async function claimInvite(tx: Prisma.TransactionClient, token: string) {
+  if (!token || !/^[a-f0-9]{32}$/.test(token)) return null
+
+  const claimed = await tx.$executeRaw`
+    UPDATE "Invite"
+    SET "usedCount" = "usedCount" + 1, "updatedAt" = NOW()
+    WHERE "token" = ${token}
+      AND "revokedAt" IS NULL
+      AND ("expiresAt" IS NULL OR "expiresAt" > NOW())
+      AND "usedCount" < "maxUses"
+  `
+
+  if (claimed !== 1) return null
+  return tx.invite.findUnique({ where: { token } })
+}