feat(sri-lankan-holidays): add comprehensive holiday management for Sri Lanka

- Introduced a new SQL migration file to populate Sri Lankan holidays from verified sources for the years 2020-2050.
- Implemented holiday data fetching and population logic in AdminCenterController and HolidayController to automatically sync holidays for Sri Lanka.
- Added a holiday data provider service to centralize holiday data retrieval and storage.
- Created a script for updating Sri Lankan holidays, including generating SQL for new years and validating holiday data.
- Developed documentation outlining the annual update process for Sri Lankan holidays, ensuring accurate data management.
- Included a JSON file containing detailed holiday information and metadata for Sri Lankan public holidays.

worklenz-backend/database/migrations/20250131000000-sri-lankan-holidays.sql

@@ -0,0 +1,60 @@
+-- ================================================================
+-- Sri Lankan Holidays Migration
+-- ================================================================
+-- This migration populates Sri Lankan holidays from verified sources
+-- 
+-- SOURCES & VERIFICATION:
+-- - 2025 data: Verified from official government sources
+-- - Fixed holidays: Independence Day, May Day, Christmas (all years)
+-- - Variable holidays: Added only when officially verified
+--
+-- MAINTENANCE:
+-- - Use scripts/update-sri-lankan-holidays.js for updates
+-- - See docs/sri-lankan-holiday-update-process.md for process
+-- ================================================================
+
+-- Insert fixed holidays for multiple years (these never change dates)
+DO $$
+DECLARE
+    current_year INT;
+BEGIN
+    FOR current_year IN 2020..2050 LOOP
+        INSERT INTO country_holidays (country_code, name, description, date, is_recurring)
+        VALUES
+            ('LK', 'Independence Day', 'Commemorates the independence of Sri Lanka from British rule in 1948', 
+             make_date(current_year, 2, 4), true),
+            ('LK', 'May Day', 'International Workers'' Day', 
+             make_date(current_year, 5, 1), true),
+            ('LK', 'Christmas Day', 'Christian celebration of the birth of Jesus Christ', 
+             make_date(current_year, 12, 25), true)
+        ON CONFLICT (country_code, name, date) DO NOTHING;
+    END LOOP;
+END $$;
+
+-- Insert specific holidays for years 2025-2028 (from our JSON data)
+
+-- 2025 holidays
+INSERT INTO country_holidays (country_code, name, description, date, is_recurring)
+VALUES
+    ('LK', 'Duruthu Full Moon Poya Day', 'Commemorates the first visit of Buddha to Sri Lanka', '2025-01-13', false),
+    ('LK', 'Navam Full Moon Poya Day', 'Commemorates the appointment of Sariputta and Moggallana as Buddha''s chief disciples', '2025-02-12', false),
+    ('LK', 'Medin Full Moon Poya Day', 'Commemorates Buddha''s first visit to his father''s palace after enlightenment', '2025-03-14', false),
+    ('LK', 'Eid al-Fitr', 'Festival marking the end of Ramadan', '2025-03-31', false),
+    ('LK', 'Bak Full Moon Poya Day', 'Commemorates Buddha''s second visit to Sri Lanka', '2025-04-12', false),
+    ('LK', 'Good Friday', 'Christian commemoration of the crucifixion of Jesus Christ', '2025-04-18', false),
+    ('LK', 'Vesak Full Moon Poya Day', 'Most sacred day for Buddhists - commemorates birth, enlightenment and passing of Buddha', '2025-05-12', false),
+    ('LK', 'Day after Vesak Full Moon Poya Day', 'Additional day for Vesak celebrations', '2025-05-13', false),
+    ('LK', 'Eid al-Adha', 'Islamic festival of sacrifice', '2025-06-07', false),
+    ('LK', 'Poson Full Moon Poya Day', 'Commemorates the introduction of Buddhism to Sri Lanka by Arahat Mahinda', '2025-06-11', false),
+    ('LK', 'Esala Full Moon Poya Day', 'Commemorates Buddha''s first sermon and the arrival of the Sacred Tooth Relic', '2025-07-10', false),
+    ('LK', 'Nikini Full Moon Poya Day', 'Commemorates the first Buddhist council', '2025-08-09', false),
+    ('LK', 'Binara Full Moon Poya Day', 'Commemorates Buddha''s visit to heaven to preach to his mother', '2025-09-07', false),
+    ('LK', 'Vap Full Moon Poya Day', 'Marks the end of Buddhist Lent and Buddha''s return from heaven', '2025-10-07', false),
+    ('LK', 'Deepavali', 'Hindu Festival of Lights', '2025-10-20', false),
+    ('LK', 'Il Full Moon Poya Day', 'Commemorates Buddha''s ordination of sixty disciples', '2025-11-05', false),
+    ('LK', 'Unduvap Full Moon Poya Day', 'Commemorates the arrival of Sanghamitta Theri with the Sacred Bo sapling', '2025-12-04', false)
+ON CONFLICT (country_code, name, date) DO NOTHING;
+
+-- NOTE: Data for 2026+ should be added only after verification from official sources
+-- Use the holiday management script to generate templates for new years:
+-- node update-sri-lankan-holidays.js --poya-template YYYY