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/src/security/user-store.js

@@ -335,6 +335,20 @@ function createUserStore(opts = {}) {
     });
   }
 
+  /**
+   * DC-052: count of users currently on this instance. Used by the
+   * license-tier gate (Free = up to 3 users, Pro = unlimited). Counts
+   * every user in users.json — including the TOTP-attributed system
+   * record (`system@totp.local`) that DC-048 bootstraps on first
+   * login. So a brand-new install always starts at count 1 (the host).
+   */
+  function countUsers() {
+    return _enqueue(() => {
+      const users = _loadUsers();
+      return users.order.length;
+    });
+  }
+
   function listAllowlist() {
     return _enqueue(() => {
       const allowlist = _loadAllowlist();
@@ -393,6 +407,7 @@ function createUserStore(opts = {}) {
     setRole,
     deleteUser,
     listUsers,
+    countUsers,
     listAllowlist,
     getUser,
     getUserByEmail,