sql first refactor

db/routines/R__fn_planning_slot_boundary_prague.sql

@@ -0,0 +1,20 @@
+-- začátek aktuálního (+offset) 15min slotu v Europe/Prague jako timestamptz (UTC instants)
+
+create or replace function ems.fn_planning_slot_boundary_prague(p_offset_slots int default 0)
+returns timestamptz
+language sql
+stable
+as $fn$
+  select (
+    (date_trunc('day', loc.ts)
+     + make_interval(
+         hours => extract(hour from loc.ts)::int,
+         mins => (floor(extract(minute from loc.ts) / 15) * 15)::int
+       )
+    )::timestamp at time zone 'Europe/Prague'
+  ) + make_interval(mins => coalesce(p_offset_slots, 0) * 15)
+  from (select now() at time zone 'Europe/Prague' as ts) loc;
+$fn$;
+
+comment on function ems.fn_planning_slot_boundary_prague(int) is
+  'Začátek 15min slotu v časové zóně site provozu (Europe/Prague floor); offset v násobcích 15 min.';