Bound session_minutes to its column so an oversized value 422s (#5)

`unifi_vlan_mappings.session_minutes` is `unsignedInteger` (ceiling 4294967295)
but the mapping validator only said `integer|min:1`. Anything larger reached the
INSERT and raised SQLSTATE 22003, so the request died as an unhandled 500 where
a validation error belongs — the same shape just fixed in central-oncall#7.
Adds `max:525600` (a year of minutes) to both storeMapping and updateMapping.

Found while fixing #5, which reported a *silent* rejection rather than a crash.
The VLAN ID validation the issue was actually about was already correct on the
server (`min:1|max:4094` on all three forms); what was missing was any way to
see it, and that half of the fix is in the shell's Unifi/Portal.vue. Sibling
numeric fields were checked at the same time: `vlan_id` (unsignedSmallInteger,
bounded 1–4094) and `max_devices` (unsignedTinyInteger, bounded 1–50) were
already safe against their columns; `session_minutes` was the only one that
could overflow.

Version 1.13.3 -> 1.13.4.

Test: dashboard-shell tests/Feature/Unifi/VlanGroupValidationTest.php

Co-Authored-By: Claude <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": "1.13.3",
+    "version": "1.13.4",
     "type": "library",
     "license": "MIT",
     "autoload": {