refactor: Move course dates from registracie to kurzy table

- Add datumOd and datumDo columns to kurzy table
- Remove datumOd, datumDo, pocetUcastnikov from registracie table
- Update schema, validators, and services accordingly
- Certificate generation now uses course dates
- Migration preserves existing data by copying most recent dates

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/db/migrations/0011_refactor_ai_kurzy_dates.sql

@@ -0,0 +1,21 @@
+-- Migration: Move dates from registracie to kurzy table
+
+-- Step 1: Add new columns to kurzy table
+ALTER TABLE kurzy ADD COLUMN IF NOT EXISTS datum_od DATE;
+ALTER TABLE kurzy ADD COLUMN IF NOT EXISTS datum_do DATE;
+
+-- Step 2: Migrate existing data - copy most recent registration dates to each course
+UPDATE kurzy k
+SET datum_od = r.datum_od, datum_do = r.datum_do
+FROM (
+    SELECT DISTINCT ON (kurz_id) kurz_id, datum_od, datum_do
+    FROM registracie
+    WHERE datum_od IS NOT NULL OR datum_do IS NOT NULL
+    ORDER BY kurz_id, created_at DESC
+) r
+WHERE k.id = r.kurz_id;
+
+-- Step 3: Drop columns from registracie table
+ALTER TABLE registracie DROP COLUMN IF EXISTS datum_od;
+ALTER TABLE registracie DROP COLUMN IF EXISTS datum_do;
+ALTER TABLE registracie DROP COLUMN IF EXISTS pocet_ucastnikov;