fix: Auto-derive region_version_id and pre-validate stat_for_unit period

Bug 1: Location saves failed with "violates check constraint
location_region_version_consistency" because the UI sends region_id
without region_version_id. Added BEFORE INSERT OR UPDATE trigger that
auto-derives region_version_id from region.version_id via PK lookup.

Bug 2: stat_for_unit inserts for periods outside the parent unit's
valid range produced a cryptic temporal FK error. Added pre-validation
in upsertTemporalRecord's INSERT branch with a friendly error message
showing the unit's actual valid range.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: jhf

app/src/app/legal-units/[id]/update-legal-unit-server-actions.ts

@@ -430,6 +430,36 @@ export async function upsertTemporalRecord({
         };
       }
     } else {
+      // Check that the parent unit exists for the requested period
+      const parentTable = unitType;
+      const { data: parentRows, error: parentError } = await client
+        .from(parentTable)
+        .select("valid_from, valid_to")
+        .eq("id", unitId)
+        .lte("valid_from", valid_from)
+        .gte("valid_to", valid_to);
+
+      if (parentError) {
+        return { status: "error", message: parentError.message };
+      }
+
+      if (!parentRows || parentRows.length === 0) {
+        const { data: allRows } = await client
+          .from(parentTable)
+          .select("valid_from, valid_to")
+          .eq("id", unitId)
+          .order("valid_from");
+
+        const rangeInfo = allRows?.length
+          ? ` The ${unitType.replace("_", " ")} exists for: ${allRows.map((r) => `${r.valid_from} to ${r.valid_to ?? "infinity"}`).join(", ")}.`
+          : "";
+
+        return {
+          status: "error",
+          message: `Cannot add ${tableName.replace("_", " ")} for the period ${valid_from} to ${valid_to}. The ${unitType.replace("_", " ")} does not exist for this period.${rangeInfo}`,
+        };
+      }
+
       let insertPayload = { ...payload, ...naturalKeyValues };
       let templateQuery = client.from(tableName).select("id").limit(1);
       for (const key in naturalKeyValues) {

migrations/20260314113637_location_auto_set_region_version_id.down.sql

@@ -0,0 +1,4 @@
+BEGIN;
+DROP TRIGGER IF EXISTS location_set_region_version_id_trigger ON public.location;
+DROP FUNCTION IF EXISTS public.location_set_region_version_id();
+END;

migrations/20260314113637_location_auto_set_region_version_id.up.sql

@@ -0,0 +1,23 @@
+BEGIN;
+
+CREATE OR REPLACE FUNCTION public.location_set_region_version_id()
+RETURNS TRIGGER
+LANGUAGE plpgsql
+SECURITY DEFINER
+SET search_path = public, pg_temp
+AS $location_set_region_version_id$
+BEGIN
+  NEW.region_version_id := (
+    SELECT r.version_id FROM public.region AS r WHERE r.id = NEW.region_id
+  );
+  RETURN NEW;
+END;
+$location_set_region_version_id$;
+
+CREATE TRIGGER location_set_region_version_id_trigger
+  BEFORE INSERT OR UPDATE OF region_id ON public.location
+  FOR EACH ROW
+  WHEN (NEW.region_id IS NOT NULL AND NEW.region_version_id IS NULL)
+  EXECUTE FUNCTION public.location_set_region_version_id();
+
+END;