feat(access): per-page user/group grants, snap-in-local

A snap-in-owned access mechanism. Adds:
  - unifi_page_grants table (nav_item_id, grantee_type, grantee_id)
    with cascadeOnDelete from nav_items so uninstalling the snap-in
    wipes its grant rows automatically
  - UnifiPageGrant model + ::userCanAccess(user, navItem) helper
  - UnifiPagesAccessController (index + update), super-admin only
  - RouteMatched listener in UnifiServiceProvider that 403s any
    unifi.* route if the matched nav_item has grants and the user
    isn't a super-admin / granted user / member of a granted group

Semantics: a page with NO grants stays open per the existing
permission middleware (no behaviour change). The moment grants are
added, ONLY super-admins and listed users/groups can see/open the
page. Super-admins always pass; their access can't be removed.

v1.4.0.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

composer.json

@@ -1,7 +1,7 @@
 {
     "name": "dashboard/unifi",
     "description": "UniFi network management, WiFi stats, and captive portal authentication for the Dashboard platform",
-    "version": "1.3.1",
+    "version": "1.4.0",
     "type": "library",
     "license": "MIT",
     "autoload": {