feat: signup toggle, invite links, admin-created users

- New invites table: token-gated signup, optional email lock,
  role/tier override, single-use, expiry.
- SIGNUPS_DISABLED site setting toggle at /admin/settings.
- databaseHooks.user.create gate in auth/server.ts blocks new account
  creation (email + Google OAuth) when disabled unless a valid invite
  cookie is present; applies invite role/tier and marks it consumed.
- /admin/invites: create/list/revoke shareable invite links.
- /admin/users: "Create user" dialog — admin sets email/role/tier,
  account is pre-verified, user gets a set-password email (admin
  never sees a password).
- Signup page reads ?invite=, validates via public
  /api/v1/invites/[token], locks the form when signups are closed
  and no valid invite is present.
- proxy.ts: allowlist /api/v1/invites/ for anonymous invite checks.

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

packages/db/src/schema/tiers.ts

@@ -8,7 +8,7 @@ import {
   unique,
 } from "drizzle-orm/pg-core";
 import { relations } from "drizzle-orm";
-import { users, tierEnum } from "./users";
+import { users, tierEnum, userRoleEnum } from "./users";
 
 export const tierDefinitions = pgTable("tier_definitions", {
   tier: tierEnum("tier").primaryKey(),
@@ -50,6 +50,26 @@ export const siteSettings = pgTable("site_settings", {
   updatedById: text("updated_by_id").references(() => users.id, { onDelete: "set null" }),
 });
 
+export const invites = pgTable("invites", {
+  id: text("id").primaryKey(),
+  token: text("token").notNull(),
+  email: text("email"),
+  role: userRoleEnum("role").notNull().default("user"),
+  tier: tierEnum("tier").notNull().default("free"),
+  createdById: text("created_by_id").notNull().references(() => users.id, { onDelete: "cascade" }),
+  createdAt: timestamp("created_at").notNull().defaultNow(),
+  expiresAt: timestamp("expires_at"),
+  usedAt: timestamp("used_at"),
+  usedById: text("used_by_id").references(() => users.id, { onDelete: "set null" }),
+}, (t) => [
+  unique("invites_token_uniq").on(t.token),
+]);
+
 export const userUsageRelations = relations(userUsage, ({ one }) => ({
   user: one(users, { fields: [userUsage.userId], references: [users.id] }),
 }));
+
+export const invitesRelations = relations(invites, ({ one }) => ({
+  createdBy: one(users, { fields: [invites.createdById], references: [users.id] }),
+  usedBy: one(users, { fields: [invites.usedById], references: [users.id] }),
+}));