feat(postcodes/AS): add American Samoa postcode (#1039)

Adds American Samoa's single ZIP 96799. Uses US ZIP system; 96799 is
the sole code for the entire territory.

Refs: #1039

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

Author: dr5hn

contributions/postcodes/AS.json

@@ -0,0 +1,10 @@
+[
+  {
+    "code": "96799",
+    "country_id": 5,
+    "country_code": "AS",
+    "locality_name": "American Samoa",
+    "type": "full",
+    "source": "manual"
+  }
+]