fix: allow retroactive first fiscal year via SIE import (#265)

The enforce_period_start_day trigger and its sie-import pre-validation
rejected any non-first-of-month period_start whenever *any other* fiscal
period existed for the company. That blocked a real user flow: after
onboarding creates a default period (e.g. current year, day 1), importing
an SIE for an older förlängt första räkenskapsår (e.g. 2017-07-28 –
2018-12-31) failed with "Non-first fiscal period must start on the 1st
of a month".

Per BFL 3 kap., the chronologically first fiscal year is the one that
may be 6–18 months and start mid-month — which is a property of *when*
the period starts relative to others, not of insert order. The trigger
and pre-validation now allow mid-month start iff no existing period
starts earlier.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

supabase/migrations/20260418120000_allow_retroactive_first_fiscal_year.sql

@@ -0,0 +1,38 @@
+-- Allow retroactive first fiscal year via SIE import.
+--
+-- The previous trigger (20260416120000) rejected any non-first-of-month
+-- period_start when *any other* fiscal period existed for the company.
+-- That blocked a common flow: user onboards and gets a default period
+-- (e.g. the current year, starting on day 1), then imports an SIE file
+-- covering an older, förlängt första räkenskapsår that starts mid-month
+-- (e.g. 2017-07-28 – 2018-12-31).
+--
+-- Per BFL 3 kap., the forst-in-time (chronologically earliest) fiscal year
+-- is the one that may be 6–18 months and start mid-month. That property is
+-- about *when* the period starts relative to other periods, not about the
+-- order rows were inserted. This migration changes the trigger to reflect
+-- that: mid-month start is allowed iff no existing period starts earlier.
+
+CREATE OR REPLACE FUNCTION public.enforce_first_of_month_for_subsequent_periods()
+RETURNS trigger
+LANGUAGE plpgsql
+AS $$
+BEGIN
+  IF EXTRACT(DAY FROM NEW.period_start) = 1 THEN
+    RETURN NEW;
+  END IF;
+
+  IF EXISTS (
+    SELECT 1 FROM public.fiscal_periods
+    WHERE company_id = NEW.company_id
+      AND id IS DISTINCT FROM NEW.id
+      AND period_start < NEW.period_start
+  ) THEN
+    RAISE EXCEPTION 'Non-first fiscal period must start on the 1st of a month';
+  END IF;
+
+  RETURN NEW;
+END;
+$$;
+
+NOTIFY pgrst, 'reload schema';