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.ics

@@ -0,0 +1,16 @@
+BEGIN:VCALENDAR
+VERSION:2.0
+METHOD:PUBLISH
+CLASS:PUBLIC
+X-WR-CALDESC:香港公众假期数据，来源：香港特别行政区政
+ 府 1823.gov.hk。
+X-WR-CALNAME:香港公众假期
+BEGIN:VTIMEZONE
+TZID:Asia/Shanghai
+BEGIN:STANDARD
+DTSTART:19700101T000000
+TZOFFSETFROM:+0800
+TZOFFSETTO:+0800
+END:STANDARD
+END:VTIMEZONE
+END:VCALENDAR