feat: add Hong Kong public holiday support

- Add scripts/fetch_hk.py to fetch HK holidays from 1823.gov.hk
- Make generate_ics() cal_name/cal_desc configurable
- Refactor update.py with REGIONS config; support --region cn|hk flag
- Generate hk/{year}.json and hk/{year}.ics under hk/ subdirectory

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hk/2027.json

@@ -0,0 +1,9 @@
+{
+    "$schema": "https://raw.githubusercontent.com/NateScarlet/holiday-cn/master/schema.json",
+    "$id": "https://raw.githubusercontent.com/NateScarlet/holiday-cn/master/hk/2027.json",
+    "year": 2027,
+    "papers": [
+        "https://www.1823.gov.hk/common/ical/tc.json"
+    ],
+    "days": []
+}