DC-052: license-tier enforcement (Free caps at 3, gates share on Pro)

First pricing enforcement. Per PRODUCT-SPEC-DECISIONS.md:
- Free = up to 3 users, no share features
- Pro = unlimited users + Tailscale-mediated share + public share links
- LIFETIME keys are creator-only (Sami runs --lifetime on his dev
  machine; production API rejects LIFETIME codes at activate())
- Free has NO trial; Pro is a deliberate paid choice

Changes:
- src/managers/license-manager.js:
  - isPro() shorthand (active + non-expired = true; LIFETIME counts)
  - allowsLifetimeLicense() reads ALLOW_LIFETIME_LICENSE env var
  - activate() rejects LIFETIME codes with a clear error unless the
    env flag is set (so Stripe webhook can't accidentally issue one)
- src/security/user-store.js: countUsers() helper for the tier gate
- src/utilities/errors.js: PaymentRequiredError (HTTP 402, code DC-402)
- routes/auth/admin.js:
  - _requireProIfUserLimitReached middleware on POST /admin/users
    and POST /admin/invites (throws 402 at count >= 3 + Free)
  - /invites/:token/accept also gated — burns the invite at cap so
    it can't be replayed later
- routes/auth/index.js: bridge licenseManager + userStore onto
  req.app.locals so the gate middleware can find them; pass
  licenseManager into the provider registry for future use

Tests: 19 new (license-tier-enforcement.test.js) covering isPro(),
allowsLifetimeLicense(), LIFETIME accept/reject paths, countUsers(),
PaymentRequiredError shape, and end-to-end admin-route gating.

Full suite: 1317/1317 passing across 50 suites.

Defer to DC-053 (share routes), DC-054 (Stripe bridge), DC-055
(pricing page), DC-056 (ToS).

dashcaddy-api/routes/auth/admin.js

@@ -29,7 +29,7 @@ const platformPaths = require('../../platform-paths');
 const { createUserStore } = require('../../src/security/user-store');
 const { createInviteStore } = require('../../src/security/invite-store');
 const emailSender = require('../../src/auth/providers/email-sender');
-const { ValidationError, NotFoundError, ForbiddenError } = require('../../src/utilities/errors');
+const { ValidationError, NotFoundError, ForbiddenError, PaymentRequiredError } = require('../../src/utilities/errors');
 const { ok, successMessage } = require('../../src/utils/responses');
 
 /**
@@ -56,6 +56,36 @@ function _requireAdmin(req, _res, next) {
   next();
 }
 
+/**
+ * DC-052: license-tier gate for user-creation endpoints.
+ *
+ * Free = up to 3 users total. Pro = unlimited. When the count would
+ * exceed the cap and the host isn't Pro, throw a PaymentRequiredError
+ * so the caller knows exactly what to do. The error message names the
+ * tier name ("Pro") so the upsell is clear.
+ *
+ * NOTE: passes through when the userStore isn't mounted (single-user
+ * installs without email auth — those don't even have /admin/*).
+ */
+async function _requireProIfUserLimitReached(req, _res, next) {
+  try {
+    const licenseManager = req.app.locals && req.app.locals.licenseManager;
+    if (!licenseManager || typeof licenseManager.isPro !== 'function') return next();
+    if (licenseManager.isPro()) return next();
+    const userStore = req.app.locals && req.app.locals.userStore;
+    if (!userStore || typeof userStore.countUsers !== 'function') return next();
+    const count = await userStore.countUsers();
+    if (count >= 3) {
+      return next(new PaymentRequiredError(
+        'Free tier supports up to 3 users. Upgrade to Pro for unlimited users.'
+      ));
+    }
+    next();
+  } catch (e) {
+    next(e);
+  }
+}
+
 function _buildEmailText({ acceptUrl, ttlHours, role }) {
   return [
     'Hi,',
@@ -134,7 +164,7 @@ module.exports = function({ asyncHandler, errorResponse, log, session, dataDir }
     return ok(res, { users });
   }, 'auth-admin-users-list'));
 
-  router.post('/admin/users', _requireAdmin, asyncHandler(async (req, res) => {
+  router.post('/admin/users', _requireAdmin, _requireProIfUserLimitReached, asyncHandler(async (req, res) => {
     const { email, role } = req.body || {};
     if (!email) throw new ValidationError('email is required', 'email');
     if (role && !userStore.VALID_ROLES.has(role)) {
@@ -195,7 +225,7 @@ module.exports = function({ asyncHandler, errorResponse, log, session, dataDir }
     return ok(res, { invites });
   }, 'auth-admin-invites-list'));
 
-  router.post('/admin/invites', _requireAdmin, asyncHandler(async (req, res) => {
+  router.post('/admin/invites', _requireAdmin, _requireProIfUserLimitReached, asyncHandler(async (req, res) => {
     const { email, role, ttlHours, sendEmail } = req.body || {};
     if (!email) throw new ValidationError('email is required', 'email');
     const ttlMs = (typeof ttlHours === 'number' && ttlHours > 0 && ttlHours <= 168)
@@ -280,7 +310,28 @@ module.exports = function({ asyncHandler, errorResponse, log, session, dataDir }
   }, 'auth-invites-peek'));
 
   // PUBLIC: accept an invite token. Creates the user, sets the session.
+  // DC-052: gated by Pro-or-room — if the user cap is hit and the host
+  // isn't Pro, reject before the user is created. The invite token is
+  // still marked used so a stale invite can't be replayed later when
+  // room opens up.
   router.post('/invites/:token/accept', asyncHandler(async (req, res) => {
+    const licenseManager = req.app.locals && req.app.locals.licenseManager;
+    const localUserStore = req.app.locals && req.app.locals.userStore;
+    if (licenseManager && typeof licenseManager.isPro === 'function' && !licenseManager.isPro()
+        && localUserStore && typeof localUserStore.countUsers === 'function') {
+      const count = await localUserStore.countUsers();
+      if (count >= 3) {
+        // Burn the invite — it can't be redeemed later under a paid tier
+        // without the host first running `addToAllowlist` to re-add the
+        // email. This prevents invite-leak spam from filling the user
+        // table and being immortalized.
+        await inviteStore.accept(req.params.token, { acceptedBy: null }).catch(() => {});
+        throw new PaymentRequiredError(
+          'Free tier supports up to 3 users. Upgrade to Pro to redeem this invitation.'
+        );
+      }
+    }
+
     const result = await inviteStore.accept(req.params.token, {
       acceptedBy: req.user ? req.user.email : null,
     });