EH-1757: remove duplicate palaute records from DB

These records have been accumulating because of mistakes in palaute
logic, but they really shouldn't be there.  Every HOKS should have one
or zero aloituskysely, one or zero päättökysely (either valmistuneet or
osia_suorittaneet) and one or zero työelämäkysely for each of its
työpaikkajaksot.

src/db/migration/V1_1736423100280__palaute_duplicate_cleanup.sql

@@ -0,0 +1,22 @@
+
+DELETE	FROM palautteet p1
+WHERE	p1.jakson_yksiloiva_tunniste IS NULL
+AND	EXISTS (
+	SELECT 1 FROM palautteet p2
+	WHERE	p1.hoks_id = p2.hoks_id
+	AND	p1.kyselytyyppi = p2.kyselytyyppi
+	AND	p2.jakson_yksiloiva_tunniste IS NULL
+	-- here we should also have a condition of having the same
+	-- rahoituskausi, but it's not needed yet because these have
+	-- accumulated only on RK 2024-2025
+	AND	p1.created_at < p2.created_at
+);
+
+DELETE	FROM palautteet p1
+WHERE	EXISTS (
+	SELECT 1 FROM palautteet p2
+	WHERE	p1.hoks_id = p2.hoks_id
+	AND	p1.jakson_yksiloiva_tunniste = p2.jakson_yksiloiva_tunniste
+	AND	p1.created_at < p2.created_at
+);
+