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/utilities/errors.js

@@ -49,6 +49,19 @@ class ConflictError extends AppError {
   }
 }
 
+/**
+ * DC-052: 402 Payment Required — used when a Pro-only feature is
+ * blocked by the license tier. Distinguishes "you need to pay" from
+ * 403 (forbidden) so the dashboard UI can render an upgrade prompt
+ * instead of a generic permission error.
+ */
+class PaymentRequiredError extends AppError {
+  constructor(message = 'Pro license required for this feature', feature = null) {
+    super(message, 402, 'DC-402');
+    this.feature = feature;
+  }
+}
+
 class RateLimitError extends AppError {
   constructor(retryAfter = 60) {
     super('Rate limit exceeded', 429, 'DC-429');
@@ -98,6 +111,8 @@ module.exports = {
   NotFoundError,
   ConflictError,
   RateLimitError,
+  // DC-052
+  PaymentRequiredError,
   DockerError,
   CaddyError,
   DNSError,