feat(access): only return granted users; add search endpoint

Listing every user in the system on the access page didn't scale —
schools have thousands of user rows. Now:
  - index() only returns users that already have a UnifiPageGrant
    somewhere. Groups stay fully listed (few of them).
  - new searchUsers(q) endpoint returns up to 20 typeahead matches
    against name or email (min 2 chars).

v1.5.2.

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": "1.5.1",
+    "version": "1.5.2",
     "type": "library",
     "license": "MIT",
     "autoload": {